This paper contains observations on 2612 Rattus rattus from buildings in the port of London, 1519 rats from ships in the docks, both samples obtained during 1941–43, and 1358 rats examined in Cyprus in 1947–48.

The sex ratio is considered in relation to the body-weight of the animals, the method by which they were killed and the locality where they were living.

The median body-weight at which female rats reach maturity in both England and Cyprus in mid-November to mid-March is approximately 97 g. The corresponding weight for the rest of the year is 87 g. in England and 78 g. in Cyprus.

The pregnancy rate is highest in rats on ships and lowest in rats in Cyprus. Seasonal fluctuations in the breeding intensity is most marked in Cyprus where it is high in April and September and lowest in January. On shore in London the year can be divided into two periods, mid-November to mid-March when the breeding rate is declining, and the rest of the year when it is higher; the peak is in September. No obvious seasonal trends in breeding activity are perceptible in rats on ships.

The average number of embryos per pregnant female is 7.5 on ships, 6.9 on shore in London and 6.1 in Cyprus. The number of embryos is correlated with the body-weight of the female, being highest in the heavier animals on both ships and shore in London, but not in Cyprus.

At least 25 per cent, of the embryos are lost between implantation and full term in rats on shore in London; the figure is probably smaller in rats on ships and in Cyprus.

The proportion of the three colour forms of R. rattus in London was similar on shore and on ships in London; this suggests that the shore population is frequently supplemented by immigrants.