PIP: The Bari, a group of Indians living in a South American rainforest, were studied in order to determine whether there were any differences in the proportions of sons and daughters at birth and/or later ages, and whether costs associated with offspring of each sex differed with unequal rates of survival as predicted by Fisher's hypothesis. Predictions derived from the Trivers and Willard model, where parents favor offspring of the sex requiring greater parental investment, were also tested. During 1988 and 1989, various Bari villages in Venezuela were visited. A total of 59 people (32 women and 27 men) thought to be in their mid-forties or older were interviewed asked to recount their life histories by questions on numbers and names of spouses and children. The dataset was complemented with genealogical and demographic information including a census taken in 1982 and information from birth records. Data were used to reconstruct the reproductive histories of 110 adults (55 women: 32 interviewed and 23 whose spouses were interviewed, and 5 men: 27 interviewed and 28 from spouses' interviews) of post-reproductive ages presumably born more than 40-50 years ago. The sex ratio at birth was approximately 1, and the relative proportions of male and female children alive were similar at ages starting at birth and ending at 15 years of age. When miscarriages and stillbirths were added to the number of live births, the proportions continued to be similar. During the prenatal stage a G-test of goodness of fit indicated that boys were more likely to die than girls. A trend, though not statistically significant, was found for higher male mortality during the 1st year of life. Sons and daughters were equally costly. Sex ratios at the end of the period of dependence were slightly biased toward daughters, confirming Fisher's hypothesis. On the other hand, no support was found for the Trivers-Willard model of sex ratios.