A temperature sensitive mutant exhibiting a very high level of chromosomal aberrations has been isolated from the Chinese hamster cell line E36. The chromosome aberrations, which include chromosome and chromatid breaks, multiradial configurations, dicentrics, and pulverizations, start to appear 1 h after a shift in temperature from 34°C to 40.5°C. The rate of sister chromatid exchange is not increased in this mutant. Analysis of somatic cell hybrids indicates that the mutation in this temperature sensitive mutant is dominan