A study has been made of the twin births in England and Wales 1952-59 and 1963-75. There is good evidence that the incidence of opposite-sexed (and by inference, dizygotic) twin births is seasonal, being about 5% higher in December than June. Triplet births seen to show a seasonal variation at about the same location and with double the amplitude. It seems likely that this seasonal variation of polyzygotic maternities is due to seasonal variation in the rates of multiple ovulation rather than to seasonal variation in any of the other reproductive parameters which could, in principle, be responsible (coital rate, spontaneous abortion, probability of fertilization). It is possible that such an hypothesized variation is due to seasonal variation in food consumption. Evidence in regard to seasonal variance in monozygotic twinning rates is equivocal but a number of considerations suggest that they too are subject to seasonal variation. If this is so, the peak incidence of MZ twin births occurs around November.