Xanthomonas campestris pv. poannua has potential as a biological control agent for perennial ryegrass (Lolium perenne L.), and it is being evaluated as a commercial bioherbicide. Field experiments were conducted on dormant `Tifway' bermudagrass [Cynodon transvaalensis Burtt-Davy × C. dactylon (L.) Pers.] and `Tifway' bermudagrass overseeded with perennial ryegrass to determine the effects of two isolates of X. campestris pv. poannua on annual bluegrass (Poa annua L.) control. Annual bluegrass control was 82% on 27 Apr. 1992 after isolate MB 218 was applied to dormant bermudagrass at 109 cfu/ml in three applications on 11 and 28 Feb. and 12 Mar. When isolate MB 245 was applied at the same rate and dates, it controlled only 60% of the annual bluegrass. The response from isolate MB 245 at the same rate and number of applications on 28 Apr. 1993 was similar to that in Apr. 1992, with 64% control on dormant turf and 52% control on overseeded turf. There was no significant advantage in annual bluegrass control when isolate MB 245 was applied at 109 cfu/ml in more than three applications during the fall and winter, compared to three applications on 15 Feb. and 1 and 11 Mar. when ratings were made on 28 Apr. 1993. The control of annual bluegrass in late Apr. 1992 and 1993 from X. campestris applied in three applications (11 and 28 Feb. and 12 Mar. 1992 and 15 Feb. and 1 and 11 Mar. 1993) at 109 cfu/ml was greater than when l08 cfu/ml was applied on the same dates.