The Anjangmui dialect group of the Kombio in Papua New Guinea has experienced a rapid increase in rural-urban migration since European contact commenced in the 1930s. Population ecology analyses of birth and migration histories for 240 Anjangmui women showed a higher total marital fertility rate in the cohort born in 1940-59 than that born in 1920-39. A decline in the age at menarche for nutritional reasons, and reduction in the birth interval for behavioural reasons, may explain the fertility difference between cohorts. Comparison of age-specific marital fertility rates between migrants in urban areas and non-migrants in rural areas revealed higher rates among migrants in the 15-19 and 20-24 year age groups, but lower rates in the 25-29 year or older age groups; the total marital fertility rate for migrants was lower than that for non-migrants. The differences may be attributable to the different influences of birth control practices on fertility reduction between the migrants in urban areas and non-migrants in rural areas. It is suggested that rural-urban migration in the Anjangmui during the post-contact period has had the effect of reducing fertility in the population as a whole.