Understanding the inheritance pattern and studying allelic relations among diverse thermosensitive genic male sterile lines (TGMS) were the objectives of this investigation. Lines F61, SA2, JP1, JP8-8-1s, ID24, and IC10, developed at the DRR, Hyderabad, and lines IR32364, IR68292, IR68294, IR68945, and IR68949, received from IRRI, Philippines, were crossed with a set of normal pollen parents, to study segregation patterns in the F2, F3, and backcross generations. The results indicated that sterility caused by the TGMS trait was governed by a single recessive gene. In segregating populations, however, individual TGMS progenies from the same F2 showed different fertility levels. This indicated that some other genes modify the major gene and affect fertility.