In the present study, genital aerobic bacterial flora was prospectively studied in 12 women receiving radiotherapy for gynecologic malignancy. Gram-positive cocci accounted for 59% and gram-negative bacilli for 27% of the 26 cervical or vaginal isolates preradiation. A similar pattern of recovery was found during external beam 60CO whole pelvis radiotherapy. The gram-positive cocci and gram-negative bacilli accounted for 42 and 46% of the pre-radiotherapy rectal isolates, respectively. There was no significant change in the pattern of rectal flora during external beam radiotherapy. Thus, results from this preliminary study would indicate that external beam radiotherapy has little or no influence on the major aerobic pathogens in either the cervix, vagina, or rectum. © 1986.