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Diurnal vanattons in spermatogenesis in the hermaphroditic snail Lymnaea stagnalis were studied by counting Sertoli cells classified according to their developmental stage (I-IV). The levels of the frequencies of these stages were studied in a field population of non-copulating snails during a 24 h period. The results were extensively analysed with statistical methods.Non-copulating snails sampled at the same time during a 24 h period appeared to be homogeneous with respect to the frequency distributions of Sertoli cells over the distinguished stages, except for the snails sampled at 5.00 a.m. This inhomogeneity coincided with lowest values or changes of environmental factors measured. The relative frequencies of stage III ('ripe' spermatids) showed a significant fluctuating pattern, whereas those of stage IV (spermiation) remained at a very constant level.

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