A survey of the gastro-intestinal nematodes in domestic sheep and goats of the Sahelian zone of Senegal was undertaken. Their prevalence and the egg production were estimated by faecal egg count in 2234 sheep and 577 goats slaughtered at the Dakar slaughterhouse during a 12 month period. The highest prevalence (%) and highest mean egg counts (EPG) were recorded at the end of the wet season. Amongst the species of nematodes encountered there are two of potential importance: Haemonchus contortus and Oseophagostomum columbianum. It is suggested that the most effective control can be achieved by a critical treatment at the end of dry season (May--June) when the EPG starts to increase, and when hypobiosis is supposed to be terminating. This would insure the greatest proportion of the existing worm population to be exposed to the anthelmintic.