The effect of irregular photoperiods on various growth parameters and oviposition of a tropical freshwater pulmonate,Bulinus tropicus, has been investigated. Over a period of 6 weeks the growth (measured as an increase in wet weight) of juvenile snails was unaffected by transfer to reversed and to random non‐circadian light‐dark regimens, although oviposition was considerably retarded in the latter experimental group and its time of onset was delayed relative to the control group held in a 12 h light‐12 h dark regimen. Snails exposed to constant light grew more, and their oviposition rate (number of eggs snail‐1week‐1) was equal to that of the control group, but these two parameters were found to be considerably reduced in snails kept in constant darkness. The food consumption, assimilation and food conversion efficiency of snails in random and reversed light‐dark regimens were comparable to the control groups, whereas animals in constant light ate more and those in constant darkness ate less. The weekly mean assimilation rate and food conversion efficiency were greater in the latter gro