Aspirated pus samples from 124 patients with peritonsillar abscess were cultured quantitatively for aerobes and anaerobes. A total of 98% of the samples yielded bacteria. Of the 550 isolates obtained (mean, 4.4 per patient), 143 were aerobes (representing 16 species or groups) and 407 were anaerobes (representing 40 species or groups). Aerobes were isolated from 86% of patients-alone in 20 cases and together with anaerobes in 87. The most common aerobic isolates were Streptococcus pyogenes (isolated from 45% of patients), Streptococcus milleri group organisms (27%), Haemophilus influenzae (11%), and viridans streptococci (11%). Anaerobes were isolated from 82% of the samples and as a sole finding from 15 abscesses. Fusobacterium necrophorum and Prevotella melaninogenica were both isolated from 38% of patients, Prevotella intermedia from 32%, Peptostreptococcus micros from 27%, Fusobacterium nucleatum from 26%, and Actinomyces odontolyticus from 23%. The rate of previous tonsillar/peritonsillar infections was lowest (25%) among patients infected with S. pyogenes and highest (52%) among those infected with F. necrophorum (P < .01). Recurrences and/or related tonsillectomies were more common among patients infected with F. necrophorum than among those infected with S. pyogenes (57% vs. 19%; P < .0001) or with S. milleri group organisms (43% vs. 19%; P < .05). beta-Lactamase was produced by only 38% of the 73 isolates of Prevotella species tested; however, 56% of the 36 patients studied harbored one or more such strains.