<jats:p><jats:bold>SUMMARY. </jats:bold> During late winter and early spring, diatoms are predominant in Lake Maarsseveen I and show a succession in blooming. To elucidate the role of parasitism in this succession, samples were taken once a week in 1978, 1979and 1980 and examined for fungal parasites, further, factors which presumably influenced the phytoplankton growth. including temperature and chemical compounds, were studied. Parasites were found on <jats:italic>Asterionella formosa</jats:italic> Hass., <jats:italic>Stephanodiscus astraea</jats:italic> (Ehr) Grun. and <jats:italic>Cyclotella comta</jats:italic> (Ehr) Kütz. The chytrid fungus <jats:italic>Zygorhizidium planktonicum</jats:italic></jats:p><jats:p>Canter was parasitic tin <jats:italic>Asterionella formosa.</jats:italic> This was the only case where high infection percentages were registered. The developmental phases of <jats:italic>Zygorhizidium plunktonicum</jats:italic> on <jats:italic>Asterionella formosa</jats:italic> were studied. The fungus was capable of affecting the spring increase and bringing about a premature end to the bloom. Only when the fungus was temporarily inhibited in its parasitic activity (this phenomenon coincided with periods of frost) was <jats:italic>A. formosa</jats:italic> able to reach a high abundance. Severe parasitism on <jats:italic>A. formosa</jats:italic> favoured the development of other diatoms (mainly of <jats:italic>Fragilaria crotoriensis</jats:italic> Kitt., <jats:italic>Stephanodiscus hantzschii</jats:italic> Grun. and <jats:italic>Stephanodiscus astraea)</jats:italic>.</jats:p><jats:p>Host‐parasite interrelationship studies performed in the laboratory showed that temperature can be an important environmental factor associated with epidemics of <jats:italic>Zygorhizidium plankiomcum</jats:italic> on <jats:italic>Asterionella formosa.</jats:italic> At very low temepratures (1.5. 1) the fungal activity was inhibited while <jats:italic>Asterionella fortnosa</jats:italic> still grew well. However, at 5, 10 and 18 the fungus manifested a high infection rate and was able to overtake A<jats:italic>sterionella</jats:italic>.</jats:p>