Morphologically aberrant colonies resulted from the infection of Acholeplasma laidlawii with two of its three known viruses and from Acholeplasma modicum cells naturally carrying virus. The patterns of colonial alteration differed between cells infected with the two A. laidlawii viruses. Colonies derived from single cells infected with the bullet-shaped virus MV-L1 (Mycoplasmatales virus-laidlawii-1) had a radial sectoring pattern of intracolonial swellings ("blebs"), whereas cells infected with the tailed icosahedral virus MV-L3 contained bubble-like blebs. Colonies from cellsinfected with the enveloped virus MV-L2 appeared identical to those obrained from uninfected cells. Aberrant colonies contained 10(6) colony-forming units of organisms and 10(6) plaque-forming units of virus serologically identical to the infecting type, indicating that both the virus and host organism were capable of simultaneous replication. Enumeration of virus by means of counting aberrant colonies was 30-fold more sensitive than infectious center assay for MV-L1 and 1.2- to 2-fold higher for MV-L3. Furthermore, blebbed colonies plaquing with a new virus specific to A. modicum. Thus, blebbing in colonies provides a valuable marker for detection of the Mycoplasmatales viruses.