Nymphs can be particularly prevelant in limestone rivers during april and may. On chalk streams and on many spate rivers the medium olive is an important summer fly. Baëtis Vernus and Baëtis tenax are very similar, and so anglers give then the same common name the Medium Olive. Peak hatches occur in May and June, but these flies are to be found on rivers until at least August. The nymphs are called agile darters.
Medium Olives frequently emerge in steady trickles rather than in batches.