Thermosensitive genic male sterility (TGMS) in rice is advantageous in hybrid seed production compared with three-line breeding. Genetic studies revealed four putative genes imparting thermosensitive male sterility in rice. It was observed that the segregation pattern of this trait in the F2, F3, and backcrosses indicated that sterility caused by the TGMS trait was controlled by a single recessive gene. It was noted, however, that individual TGMS segregants drawn from the same F2 population exhibited a differential pattern for fertility alteration, suggesting the influence of modifier genes. This makes the transfer of this character difficult to achieve, as selection has to be made for the appropriate fertility alteration level in addition to selection for the TGMS trait.