Abstract. 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. Key words: High fecundity, Mice, Polyovular follicles (POFs) (J. Reprod. Dev. 56: [449][450][451][452][453] 2010) n normal adult mouse ovaries, follicles contain a germ cell surrounded by one or more layers of granulosa cells, according to the follicle developmental stage. However, follicles with more than one oocyte are rarely found [1]. During initial development, multiple germ cells are present within the individualizing follicles, although in most cases, excess cells are eliminated and mammalian ovarian follicles contain only one germ cell per follicle. Follicles with more than one oocyte are referred to as multi oocyte follicles (MOFs) or polyovular follicles (POFs). They have been reported to occur in the ovaries of humans [2], primates [3], dogs [4,5], cats [6], goats [7] and mice [8]. In rats and mice, the incidence of POFs increases when the animals are exposed to estrogen analogs such as diethylstilbestrol, estradiol benzoate or genistein in a dose-dependent manner [9,10]. The POFs observed in mice and humans have been postulated to be remnants of oocyte clusters that did not separate, and which have become enclosed in follicles during neonatal primordial follicle assembly [2,10,11].Fecundity (the number of offspring per litter) can be related to an increase in the number of ova ovulated or in the efficiency of fertilization and embryo development. Booroola sheep have multiple offspring because of an increase in the number of follicles ovulated, which is related to the fecundity gene FecB, a mutation in bone morphogenetic protein receptor IB. The presence of FecB increases litter size by one or two extra lambs [12]. Polyovular follicles have been especially noted in bitches [13]. The presence of POFs could affect the number of oocytes recovered in assisted reproductive protocols and may also influence the ovulation rate and prolificity of these animals [5].Previous analysis of fecundity traits revealed that in high fecundity mouse lines (FL1, FL2), the number of ova ovulated is higher than the number of corpora lutea (CL) present. This is particularly evident in the FL1 line [14]. This observation suggests that individual follicles release more than one oocyte. The occurrence of polyovular follicles in specific strain...