SUMMARYThe activity of the primary and secondary zoöspores in four genera of the Saprolegniaceae was studied and compared at temperatures from 5°to 35°C. Under carefully controlled conditions, primary zoöspores, for example, swarmed for a shorter period, carried on normal activity over a narrower temperature range, moved at a slower rate, and had a higher temperature coefficient than the secondary. Since the two types have different“temperature characteristics,”their activities are probably controlled by different catalysts.