Quantitative data are presented on the depth-time distribution of four cryptophycean species (Cryptomonas marssonii, C. cf. ovata, C. pusilla and Rhodomonas minuta var. nannoplanktica) over a three year investigation period in Mondsee, Austria, a deep, stratifying alpine lake. Net rates of population increase and decrease are calculated and related to environmental variables (temperature, light, turbulence) and impact by herbivorous zooplankton. Although cryptophyceans rarely comprise more than 15 – 20% of total phytoplankton biovolume they contribute considerably to plankton community dynamics.