Cyclopoid copepods were collected over a period of 17 months from six man-made water reservoirs in an area of northern Ghana where dracunculiasis is endemic. The numbers and species of cyclopoid copepods as well as the species naturally infected with Dracunculus medinensis larvae were recorded at fortinightly intervals. The occurrence of copepods was compared with concurrent rainfall and with monthly incidences of dracunculiasis in the year after the reservoir survey. Seven species and two subspecies of copepods were found in the study area: Thermocyclops neglectus decipiens, T. crassu' consimilis, T. incisus, T. inopinus, T. emini, Mesocyclops major, M. ogunnus, M. kieferi and Cryptocyclops linjanticus. More than 60% of all the cyclopoids found were T. n. decipiens and only four species (T. incisus, T. inopinus, T. n. decipiens and M. kieferi) were found infected. Infection rates were very low (< or = 0.3%). The seasonal fluctuations of cyclopoid copepods showed different patterns in the six reservoirs, perhaps related to local circumstances not investigated in the present study. As the people in the study area are very mobile during the dracunculiasis transmission season, it seems likely that some of them become infected at water sources other than the ones investigated. It was therefore difficult to correlate occurrence of cyclopoid copepods in the local water sources with the prevalence of the human infection in the catchment area. No conclusion as to the most important intermediate host(s) in the area could be made.