The first objective of this study was to estimate the seroprevalence of ovine paratuberculosis in the Basque Country of Spain. Taking as population all the flocks with more than 100 sheep, about 4504 sheep (2.5% of the total number of heads) in 226 flocks (27.8% of the flocks) were sampled, and sera subjected to Agar Gel Immunodiffusion (AGID) and ELISA. Seroprevalence was found to be 31.4% of the flocks (±5.6) in AGID, and 23.5% (±5.3) in ELISA. 5.8% (±0.7) of the sheep gave positive results in the ELISA and 2.0% (±0.5) in the AGID. The aims of the second part were to investigate the possible association between serology and production factors and to evaluate the effects of vaccination in 10 sheep flocks (about 1400 sheep) with a previous history of clinical cases. About 50% of the adult sheep in each flock, and 50% of the replacer lambs of each year, were vaccinated. The follow-up included a yearly serological study of the whole flock, analysis of the individual productive data, and evaluation of the immunological status of replacers, before and after vaccination. We have found an association between ELISA response and milk production, that, in spite of the influence of other factors, can be estimated in about 20 liters per ELISA index unit. In positive and negative terms, the difference would be of at least a 7.1%. On the other hand, we have been unable to demonstrate a consistent association between seropositivity in the ELISA and fertility. Periodic oscillations of an annual cycle in the reactivities in the ELISA have been observed, which could be related to physiological changes associated with the reproductive cycle, although they cannot be firmly established because of the methodology employed in this study. Moreover, a high proportion (62%) of the sheep tested in the four year follow-up seroconverted at any time in the ELISA, while almost all of the animals with a positive result in the AGID were not present in the following year. A significantly higher proportion of animals vaccinated at birth remained in the flock in the second year compared to the controls (22% vs. 10%; p less than 0.05). This suggests a protective effect of vaccination reaching beyond the traditionally accepted period for the manifestations of paratuberculosis, which could already be present at an early age. The same effect appeared conditioned, in sheep vaccinated in adulthood, by age at vaccination. These results confirm the existence of important losses associated with paratuberculosis but without the typical signs of the disease.