The Everglades region of southern Florida is a large, subtropical, freshwater marsh prone to periodic droughts. Usually about 70% of the annual rainfall occurs between June and October (Leach -- et al., 1979), and water levels increase during this period. The marsh gradually dries until the next rainy season begins again the following spring. Small fishes of the families Cyprinodontidae and Poeciliidae are an important link in the Everglades food web (Kushlan, 1976), but information about their biology in southern Florida is limited. The effect of seasonally fluctuating water levels on growth, the length of time that individuals remain in the population, the age at which they become available to predators, and the age at maturation must be determined to accurately assess the role of these fishes in the Everglades ecosystem. The first step to answer these questions is age determination. Most cyprinodontids and poeciliids reproduce during much of the year in the Everglades (Loftus and Kushlan, pers. comm.), and the results of age and growth determination by length-frequency analysis have been generally unsatisfactory. The best way to determine the age of these small fishes appears to be by the counting of daily growth increments in the otoliths.