Abstract.1The egg masses ofSimulium ornatum, S.equinumandS.vernumare described and information is given on numbers of eggs per egg mass and the size and number of batches laid by individual females.2The oviposition sites were studied in relation to water velocity, wind speed and direction.3The incubation periods at different river temperatures and the effects of desiccation on percentage hatch and incubation times were investigated.4The numbers of eggs laid were assessed for the winter generations ofS. ornatumandS.equinum.The relevance of these to the overall production estimates is discussed