The accumulation of acid phosphatase activity has been studied during Ca2+-induced sporangium formation in the water mold Achlya bisexualis. The acid phosphatase activity is localized in a subfraction of the vegetative mycelia that has the characteristics of a lysosomal fraction. Vegetative hyphae possess high levels of acid phosphatase activity which increase 2- to 3-fold during the developmental sequence. Actinomycin D at low doses (10 μg/ml) prevents accumulation of the enzyme activity suggesting that RNA synthesis is essential for the increase in acid phosphatase activity. Coincident protein synthesis may give rise to the enzyme increase since cycloheximide (100 μg/ml) added at sequential intervals prevents further accumulation of enzyme activity. Vegetative hyphae possess only one detectable form of acid phosphatase but as development progresses a new electrophoretically faster band of activity appears which finally becomes predominant. Acid phosphatase activity also accumulates extracellularly during the differentiation process.