A total number of 18.675 sera of pigs were examined for the presence of antibodies to various serotypes of Leptospira in the Netherlands during the period from 1975 to 1980. The study showed an average of 21 per cent of positive serological findings. The distribution of these findings amongst the various serotypes of Leptospira was as follows: L. bratislava 39%; L tarassovi 22%; L. ballum 15% and L. icterohaemorrhagiae 12%. The combined other serotypes scored approximately 12%. The percentage of serologically positive animals having a history of abortion was approximately 23%. 23% up to 1978. In 1979, it fell to 11%, and it increased again to 32% in 1980. The common serotypes of Leptospira in sows which aborted were L. bratislava, L. tarassovi, L. ballum and L. icterohaemorrhagiae. L. pomona, L. australis, L. canicola and L. hardjo were rare but were identified in a few cases. It is recommended that measures should be adopted to prevent the introduction of these serotypes.