Strict anaerobic culture techniques were used to quantitate heterotrophic bacteria present in hindguts of Reticulitermes flavipes. The grand mean number of viable cells per hindgut was 0.4 X 10(5) (first-instar larvae), 1.3 X 10(5) (third-instar larvae), 3.5 X 10(5) (workers), and 1.5 X 10(5) (soldiers). Of a total of 344 isolates, 66.3% were streptococci that were always obtained regardless of the origin of termites, their developmental stage or caste, or their length of captivity. Most of the remaining isolates were strains of Bacteroides and Enterobacteriaceae. A small percentage were strains of Lactobacillus, Fusobacterium, and unidentified anaerobic gram-positive rods. Recovery of bacteria from worker hindguts was 13.0% of the direct microscopic count. Isolations performed aerobically failed to reveal strict aerobes. Attempts to isolate cellulolytic bacteria were uniformly unsuccessful. Of 145 streptococcal strains isolated from freshly collected termites, almost all were Streptococcus lactis and S. cremoris. Enterobacteriaceae isolates from the same termite specimens were indole-positive Citrobacter, citrate-negative Citrobacter, and Enterobacter cloacae. The possibility of in situ interspecies lactate transfer, between lactate producers (e.g., streptococci) and lactate fermenters (Bacteroides), is discussed.