Males of Anopheles albimanus were exposed to X-rays (2968 R or 5088 R), and chromosome aberrations in their progeny were identified cytologically. A total of 1121 treated sperm were assayed and 277 aberrations were observed as follows: 148 translocations, 67 pericentric inversions, and 62 paracentric inversions. No attempt was made to record deletions, duplications or other more minor changes in chromosome structure. More Y-linked translocations were obtained from males that were irradiated when they were 24-48 hr old than from males that were 0-24 hr old.