diff --git a/src/blocks/scratch3_pen.js b/src/blocks/scratch3_pen.js
index 5755af78b..b2f7d4b6d 100644
--- a/src/blocks/scratch3_pen.js
+++ b/src/blocks/scratch3_pen.js
@@ -55,7 +55,7 @@ class Scratch3PenBlocks {
             penDown: false,
             hue: 120,
             shade: 50,
-            transparency: 100,
+            transparency: 0,
             penAttributes: {
                 color4f: [0, 0, 1, 1],
                 diameter: 1
@@ -169,7 +169,8 @@ class Scratch3PenBlocks {
     }
 
     /**
-     * Update the cached RGB color from the hue & shade values in the provided PenState object.
+     * Update the cached color from the hue, shade and transparency values in the provided
+     * PenState object.
      * @param {PenState} penState - the pen state to update.
      * @private
      */
@@ -184,7 +185,7 @@ class Scratch3PenBlocks {
         penState.penAttributes.color4f[0] = rgb.r / 255.0;
         penState.penAttributes.color4f[1] = rgb.g / 255.0;
         penState.penAttributes.color4f[2] = rgb.b / 255.0;
-        penState.penAttributes.color4f[3] = penState.transparency / 100.0;
+        penState.penAttributes.color4f[3] = this._transparencyToAlpha(penState.transparency);
     }
 
     /**
@@ -198,17 +199,39 @@ class Scratch3PenBlocks {
         if (value < 0) value += 200;
         return value;
     }
-    
+
     /**
-     * Wrap a pen transparency value to the range (0, 100).
-     * @param {number} value - the pen transparency value to the proper range.
-     * @returns {number} the wrapped value.
+     * Clamp a pen transparency value to the range (0,100).
+     * @param {number} value - the pen transparency value to be clamped.
+     * @returns {number} the clamped value.
      * @private
      */
-    _wrapTransparency (value) {
-        value = value % 100;
-        if (value < 0) value += 100;
-        return value;
+    _clampTransparency (value) {
+        return MathUtil.clamp(value, 0, 100);
+    }
+
+    /**
+     * Convert an alpha value to a pen transparency value.
+     * Alpha ranges from 0 to 1, where 0 is transparent and 1 is opaque.
+     * Transparency ranges from 0 to 100, where 0 is opaque and 100 is transparent.
+     * @param {number} alpha - the input alpha value.
+     * @returns {number} the transparency value.
+     * @private
+     */
+    _alphaToTransparency (alpha) {
+        return (1.0 - alpha) * 100.0;
+    }
+
+    /**
+     * Convert a pen transparency value to an alpha value.
+     * Alpha ranges from 0 to 1, where 0 is transparent and 1 is opaque.
+     * Transparency ranges from 0 to 100, where 0 is opaque and 100 is transparent.
+     * @param {number} transparency - the input transparency value.
+     * @returns {number} the alpha value.
+     * @private
+     */
+    _transparencyToAlpha (transparency) {
+        return 1.0 - (transparency / 100.0);
     }
 
     /**
@@ -315,6 +338,7 @@ class Scratch3PenBlocks {
         } else {
             penState.penAttributes.color4f[3] = 1;
         }
+        penState.transparency = this._alphaToTransparency(penState.penAttributes.color4f[3]);
     }
 
     /**
@@ -386,7 +410,7 @@ class Scratch3PenBlocks {
         const penAttributes = this._getPenState(util.target).penAttributes;
         penAttributes.diameter = this._clampPenSize(Cast.toNumber(args.SIZE));
     }
-    
+
     /**
      * The pen "change pen transparency by {number}" block changes the RGBA "transparency" of the pen.
      * @param {object} args - the block arguments.
@@ -395,10 +419,10 @@ class Scratch3PenBlocks {
      */
     changePenTransparencyBy (args, util) {
         const penState = this._getPenState(util.target);
-        penState.transparency = this._wrapTransparency(penState.transparency + Cast.toNumber(args.TRANSPARENCY));
+        penState.transparency = this._clampTransparency(penState.transparency + Cast.toNumber(args.TRANSPARENCY));
         this._updatePenColor(penState);
     }
-    
+
     /**
      * The pen "set pen transparency to {number}" block sets the RGBA "transparency" of the pen.
      * @param {object} args - the block arguments.
@@ -407,7 +431,7 @@ class Scratch3PenBlocks {
      */
     setPenTransparencyTo (args, util) {
         const penState = this._getPenState(util.target);
-        penState.transparency = this._wrapTransparency(Cast.toNumber(args.TRANSPARENCY));
+        penState.transparency = this._clampTransparency(Cast.toNumber(args.TRANSPARENCY));
         this._updatePenColor(penState);
     }
 }