A survey of swine destined for slaughter in Manitoba was conducted to examine the incidence of ochratoxin A (OA) in swine herds from different regions of Manitoba throughout the year 1989–90. Thirty‐six percent of the serum samples which were collected from 1600 pigs contained detectable levels of OA. The identity of this toxin was confirmed using liquid chromatography‐mass spectrometry and enzymatic hydrolysis. There was a significant effect of the region from which the herds originated, as well as the season in which the samples were collected on both the incidence(p <0.001) and concentration of OA (p < 0.001). In July, 65% of the samples contained detectable levels of OA, compared with 38, 21 and 17%, in April, October and January respectively. Furthermore, 24% of the samples collected in July contained greater than 15 ng/ml of OA, while only 2, 9, and 1% of the samples collected in April, October and January respectively, contained greater than 15 ng/ml of OA. Based on the six samples collected from each herd, it appears that the presence and concentration of OA within a herd may be estimated from a limited number of animals per herd.