The relationship between female age and infertility is examined using a single-island Micronesian population case. Demographic data, derived primarily from reproductive history interviews, show that a significant age-associated decline in marital reproductive performance is absent before women reach their late thirties in this population but a substantial decline is present once women reach their forties. Ethnographic data support the demographic inference that couples are maintaining relatively high levels of conjugal coital activity with both advancing female age and increasing marital duration. Thus coital activity levels appear to be an important factor in the maintenance of fertility in this group before the mid-thirties but decreases in fecundability after this age are due primarily to reductions in fecundity, not to declines in coital activity. The description of the Butaritari case lends support to Underwood's (1990) suggestion that a "Micronesian pattern" of reproductive performance may exist for the region's atoll-based populations and underscores the promise of further investigations of these special cases in the fields of demography and reproductive ecology. :The aim of this study of infertility was to describe demographic patterns of infertility by age and their link with age-associated behavior changes, such as coital activity, on the Butaritari Atoll north of Tungaru in the Republic of Kiribati. The population is relatively circumscribed on this high-population-density island; there were high rates of nuptiality, minimal premarital exposure to pregnancy risk, low contraceptive usage, and an absence of sexually transmitted diseases that cause infertility. The population in 1990 was 3800, of which almost 50% were aged 15 years. Fertility was high, at a mean of 4.5 births for ever married women (N=791) and a completed fertility of 7.14 for ever married postmenopausal women. Almost all men and women marry. Primary infertility levels were only 1.29% of ever married postmenopausal women (N=231). Data on reproductive histories were collected from 87.1% (N=203) of all women aged 15 years in the village in 1990/91 and supplemented with other archival records. A subsample of 34 women were interviewed for a detailed analysis of life course changes in reproductive and sexual behavior. Census data from 1990 were also collected and checked against vital registration records. The following measures were examined: infertility in relation to marriage duration, current reproductive status of women in relation to age, infertility in relation to parity, infertility in relation to age, pregnancy failure in relation to age, and ethnographic factors. The findings indicated that infertility rarely occurred before the late 30s because of the low levels of subfecundity, no age variation in breast feeding patterns, no significant increase in reported fetal death rates before the 40s, and sustained coital frequency with increasing marriage duration. Coital frequency declined after the age of 40 years, as did fecundity. The coital patterns were found to be similar to the Micronesian region; women produce as many children at later ages as they do at earlier ages.