In Gatineau Park near Ottawa Simulium venustum adults were present in appreciable numbers around man (usually 20-100 flies/man/hr.) only between mid-May and mid-June in 1958-60, numbers after late June being negligible apparently because a succeeding generation failed to mature in significant quantity. It is suggested that differences in the duration of the S. venustum biting problem in different parts of eastern Canada largely depend on whether or not streams remain suitable for the aquatic stages after the spring generation derived from overwintered eggs has emerged.

In sweep-net catches of flies attracted to man at fixed sites the proportion of parous flies was nil in mid-May and increased to near 100 per cent by mid-June, 3 change sometimes interrupted by the appearance of a second wave of nulliparous individuals from late fly-producing streams. The latter were anatomically indistinguishable from flies produced earlier apart from reduced body size. The results suggest that the populations studied were anautogenous and that there was no deviation from this condition that could be attributed to variations in the food available for larvae in different streams or at different times.

The proportion of attracted parous flies containing relict eggs sometimes tended to increase towards the end of the season, suggesting that a large proportion of the few flies still remaining had completed two or more ovarian cycles.

Parous flies tended to be particularly active in late afternoon and evening but instances when this diurnal pattern did not occur were observed.