Fasciola hepatica infection is an important cause of lost productivity in livestock worldwide. Effective control of fasciolosis is difficult, especially in milking cows, which can only be treated during dry periods, a control strategy that has not been yet evaluated. In this cross-sectional study, we investigated the effect of the type of flukicide treatment on the prevalence and intensity of infection in dairy cattle from Galicia, an area where fasciolosis is endemic and which is also the main milk-producing region in Spain. Faecal samples were taken from 5188 dairy cows on 275 randomly selected farms for measurement of the concentration of F. hepatica coproantigens by a monoclonal antibody based immunoassay (MM3-COPRO ELISA). On the same day as the sampling, each farm owner/manager was questioned about the types of treatment used on the farm. Three groups of farms were considered according to the fasciolicide treatment: (A) flukicides were not used, (B) an anthelmintic effective against mature stages of flukes was used (albendazole or netobimin) and (C) a fasciolicide effective against immature and mature stages was used (triclabendazole: TCBZ). Results indicated that 16.0% (832/5188) cows from 61.1% (168/275) herds were infected by F. hepatica. The mean coproantigen concentration in infected herds was 13.0ng/ml (range 0.9-112.6ng/ml). The highest individual concentration recorded was 496.6ng/ml. Herd and within-herd prevalences of F. hepatica were similar in all three groups, but surprisingly, individual prevalence and antigen concentration were higher in Group C (p<0.05). The percentage of farms with within-herd prevalences >25% was very high in all three groups, and no significant differences were observed. In contrast, the percentage of herds with mean antigen concentrations >20ng/ml was significantly lower (p<0.05) in Groups A and B (14.4% and 14.9%, respectively) than in Group C (50.0%). The proportion of herds that exceeded both limits (25% for prevalence and/or 20ng/ml for coproantigen concentration) was also significantly higher (p<0.05) in Group C than in untreated animals (Group A). The survey showed that most dairy farmers are unaware of the existence of F. hepatica infection on their farms, and treatments, when given, are administered without prior diagnosis. Treatment with TCBZ administered only at drying off did not show advantages over other measures including no treatment, or treatment with other benzimidazoles. Consequently, TCBZ should only be used to treat individual animals after correct diagnosis of the infection, and correct management measures taken to control re-infection.