An adult Bangladeshi woman had persistent bloody diarrhea. Repeated stool cultures yielded Plesiomonas shigelloides in pure growth. Tissue specimens of the colon were consistent with pseudomembranous colitis. Treatment with tetracycline, to which the isolate was susceptible, brought prompt recovery; the stool cultures became negative and the serum antibody titer against P. shigelloides lipopolysaccharide, as measured by hemagglutination inhibition with P. shigelloides lipopolysaccharide-sensitized sheep erythrocytes, declined from 1:160 to 1:40.