After 15 yr tagging at the nesting ground of Chelonia mydas at Tortuguero, Costa Rica, 447 remigration returns have been recorded. These confirm previous indications that some Tortuguero females return to nest every 2 yr and some every 3 yr, with the latter being the more frequent cycle. Returns after absences of 4 yr are so frequent that it seems probable that a regular cycle of this magnitude also exists. Longer absences recorded (up to 9 yr) probably can be attributed to missed encounters or to weak site tenacity by the returning turtles. A growing list of two—time and three—time returns shows that although an individual usually maintains a constant cycle, modulation of this may occur, and that the change may be either from 3 to 2 yr, or vice versa. It is suggested that the modulation reflects ecological conditions on the feeding ground. A sudden extensive shift to a 2—yr cycle, by turtles that had been nesting on a 3—yr cycle, probably contributed to the heavy nesting that occurred in 1969, after several previous weak seasons.