Volumetric reduction of nest contents prior to hatchling emergence was deter- mined for natural hatchery nests of loggerhead sea turtles by implantation of a disc apparatus. The hypothesis that significant reduction occurs at hatching has been falsified. Reduction of nest contents occurred prior to hatching due to a loss in egg turgor. Initial reduction occurred in most nests within 7 to 8 days of emergence. The maximum depth of air space created above the eggs was 4.0 + .26 cm (x + SE, n = 52). Amount of egg-mass reduction was significantly correlated to the number of hatchlings and full-term embryos per nest and not correlated to egg number. The rate of volumetric reduction varied seasonally and can be explained by temperature. * * * Incubation length of eggs in natural turtle nests is usually defined as the elapsed time between oviposition and hatchling emergence. This time period includes embryonic development, hatching, possible delayed emergence, and digging to reach the nest surface. Determination of the time required for these events has been made by frequent nest openings. This practice, however, may decrease developmental time if the eggs are moved (Bustard, 1972). Hatchlings of Pseudemys scripta troostii (=elegans) in Louisiana, require up to 5 days for yolk retraction and a concomitant increase in size and firmness of the shell while remaining in the nest (Cagle, 1950). Similar results were found for artifically incubated eggs of Terrapene ornata orata (Legler, 1960). In natural nests of Malaclemys terrapin terrapin on Little Beach Island, New Jersey, hatching requires up to 4 days while emergence occurs 1 to 9 days later (Burger, 1976). The sequence of post-hatching events was observed for one green turtle, Chelonia mydas, clutch from Tortuguero, Costa Rica, trans- planted to a sand-filled wooden box with a glass side (Carr and Ogren, 1960). The eggs began to lose turgor 4 days prior to hatching. Air space above nest contents was created during hatching. Vertical movement of turtles began 2 to 3 days after hatching and emergence occurred 7 days after most of the eggs hatched. The production of an air space between the egg-mass and ceiling of the egg cavity during hatching is well documented for sea turtle nests (Carr and Hirth, 1961; Hendrickson, 1958; Schulz, 1975). The length of time for hatching has not been determined for natural nests. Early hatching