From March 2001 to February 2004, 724 faecal samples of cattle under field conditions were collected in 121 farms from Lugo (NW Spain), an area with oceanic climate. The risk periods of infection by Paramphistomum trematoda in cattle was assessed by analyzing the prevalence of paramphistomosis, egg-output and climatic data. A significantly higher percentage of cattle passing trematode eggs by faeces was recorded firstly in September and secondly in March. The greatest numbers of eggs were achieved in May. Finally, the individual average prevalence was 19% and 36% of the farms had Calicophoron daubneyi-positive cattle. We conclude that the periods of high risk for C. daubneyi infection in areas with oceanic climate are those following periods with elevated rainfall, which occurs in May-June and November-December in these areas. Appropriate measures for reducing the risk for paramphistomosis are needed and the administration of chemotherapy with efficient anthelmintic in June and in November is strongly recommended.