Sixty-eight night surveys of American alligators (Alligator mississippiensis) were conducted from 17 June 1976 to 12 July 1977 on a large cypress-fringed lake in north-central Florida. Multiple regression analyses of the effects of II environmental variables (water temperature, air temperature, wind speed, wave height, cloud cover, water level, moonlight, precipitation, 24-hour precipitation, 24-hour maximum temperature, and 24-hour minimum temperature) on surveys with a white light (n = 44) indicated that counts were positively correlated with water temperature and negatively associated with water level. Water temperature was the most important variable in cool weather (I October - 1 May) and accounted for 85% of the variation in counts (n =22) during those months. Water level was the most important variable in warm weather (I May - I October) and accounted for 53% of the variation during those months. Monthlychanges in size composition and distribution of the alligator population are described and their relationships to night-light counts are discussed. No significant differences were.detected in the effectiveness of airboats relative to outboard motorboats or red light relative to white light in counting or approaching alligators. Light intensity had a significant effect on both counts (P <0.02) and on approachability (P <0.05). Guidelines are established for conducting future night-light surveys for population trends.