The aim of this study was to evaluate the prevalence of follicles with more than one oocyte (polyovular follicles, POFs) in mouse lines selected for high fecundity. The ovaries of 18 mice, 6 each from 3 different lines, were examined to evaluate the number of POFs and the follicular histology. Polyovular follicles were observed in the two high fecundity breeds, FL1 and FL2, as well as in the unselected control line, DUKsi. The highest number of POFs per ovary (27.0 +/- 7.2) was found in the FL1 line. The FL2 and DUKsi lines had 1.9 +/- 0.7 and 0.6 +/- 0.3 polyovular follicles per ovary, respectively. Most of the POFs contained 2 oocytes (>80%), but occasionally follicles containing up to 7 oocytes were observed. Follicles with more than 2 oocytes were observed in the FL1 line only.