This is the first part of an account of seasonal fluctuations of the Asterionella population in the plankton of certain lakes in the English Lake District. It records the results of regular, usually weekly, estimations of diatom nos., and concns. of nitrates, phosphates and silicates in the water. Reference is made to other factors such as competition for nutrients between the various plankton diatoms, fungal parasitism, rainfall and light penetration. For 2 lakes and the s. basin of the 3d, the records cover 4 or 5 yrs. In the n. basin of the 3d lake, the observations, in varying degrees of detail, cover a period of 18 yrs. In most yrs. the max. density of Asterionella was reached when the silica concn. fell to approx. 0.5 mg. per l.; a catastrophic drop in nos. usually follows. No large diatom populations were produced at concns. of silica below 0.5 mg. per 1. The changes in the concns. of nitrate N and phosphate P bear no evident relation to those in the nos. of Asterionella. Phosphate P is usually approx. 0.001 mg. per l.