Seasonal fluctuations in the farrowing rates (FR( of sows and gilts were determined by analysis of breeding records from 11 herds, covering a total of 26 herd-years of data. Of 22,997 sow and gilt matings, 72.7% resulted in farrowing. For matings during July, August, and September, 60.8% of females farrowed, compared with 76.9% for matings during the remainder of the year. The FR was higher in multiparous sows than in primiparous sows and was lowest in nulliparous gilts. The FR was 65.5% for gilts bred in confinement and for gilts maintained outside. The seasonal fluctuation in FR was similar for confinement and outside-housed gilts. Water sprinkling or evaporative cooling methods did not prevent seasonal fluctuations in the FR of sows or gilts. For sows confined to crates after breeding, the FR was 13.8% higher than for sows in group-housing systems, but the seasonal fluctuation in FR was not substantially altered. It was concluded that the FR of female swine is markedly influenced by season of the year and by parity. The seasonal fluctuations in FR occurred in group- and individually housed swine and in confinement and outside-bred gilts. Water sprinkling or evaporative cooling systems, as used in the herds investigated, appeared to be of questionable value in preventing infertility during the warm months.