The transmission of bovine Onchocerca spp. in Britain was investigated by studying the Ceratopogonidae and Simuliidae which attacked bait cattle between April and October at two sites in North Wales, where either Onchocerca lienalis and O. gutturosa (Cynwyd), or O. lienalis alone (Pentrefoelas) occurred. Based on the relative abundance of each species, their seasonal variation in abundance and preferential feeding sites on cattle, the presence of Onchocerca sp. infective larvae and the development of patent infections in bait cattle, vectors were identified as Simulium ornatum s.1. at Pentrefoelas and S. reptans at Cynwyd (0·5% and 1·5% of parous flies infective, respectively). There was no significant difference between the lengths of the infective larvae from vectors at either site (Pentrefoelas: mean = 543·5 μm, SD = 29·45; Cynwyd: mean = 550·86 μm, SD = 35·26; p > 0·1), which were consistent with descriptions of O. lienalis. The identity of a vector for O. gutturosa was not determined and the role of certain candidates is discussed. In this respect, the absence of Culicoides nubeculosus was notable. Transmission of Onchocerca sp. was limited to a short period of about 8 weeks, by a combination of vector abundance and parasite uptake from dermal skin layers, and a mean daily temperature requirement of 15°C or more for complete parasite development in the vector. The effect of this temperature restriction in relation to the geographical distribution of the two species is discussed.