We studied nesting ecology and sex determination in a population of Macrochelys temminckii (Alligator Snapping Turtle) inhabiting the lower Apalachicola River in western Florida. During 1990–1991, we located 105 nests (63 intact, 42 depredated); 83% occurred on prominent dredged spoil substrates. Coupled with data from additional years, mean clutch size of 130 nests was 35.1 (min–max = 17–52). Most nesting occurred from 20 April to 18 May, with about a week annual variation. Hatching extended throughout August. In nests that we protected from predators, hatching success averaged 78% (24 nests) in 1990 and 66% (26 nests) in 1991. Overall sex ratios were roughly 1 male: 2 females, although individual nests varied from all males to all females in conjunction with solar exposure.