Wives in monogamous unions are markedly more fertile than their counterparts in polygynous unions among the Mende of rural upper Bambara Chiefdom, eastern Province, Sierra Leone. The study controls for ethnicity of both husband and wife and for marital history of the latter. The data reported were obtained through a comprehensive census of 9 rural communities. The study has no inherent sampling bias other than possibly in the selection of the study communities. Because a universe rather than a sample is being analyzed, tests of statistical significance are superfluous. Differences appearing in the dimensions measured within the 9-village universe are automatically significant. The aggregate monogamous-polygynous fertility differential is explained partially in terms of patterns of remarriage among widows and divorcees and partially by the differential effects of marital structure (monogamy vs. polygyny) upon female fertility.