Analyses of vital data derived from a family record register for the native population of Guam reveal significant variations in births, deaths, and marriages over the period 1901-41. Although lacking marked photoperiod or temperature changes of temperate zones, the tropical island is subject to marked seasonal differences in rainfall characteristic of western Pacific islands. Marital patterns exhibit troughs associated respectively with the Lenten period and with Christmas celebrations. Infant and childhood deaths show close correspondence with rainfall patterns, consistently exceeding expected values during the rainy season (July-November) when conditions are optimal for the spread of communicable and gastrointestinal diseases. Births attain a peak in November, or at the beginning of the more advantageous season for infant health and survival. Seasonality in vital events, reported for many Euroamerican and some African and Asian populations of modern and historical periods, has rarely been documented for native populations of the tropical Pacific. Comparisons of differences in these patterns among different populations in varied environments provide unique opportunities to evaluate causal models of interactions among biological, sociocultural, and physioenvironmental factors.