Northern krill, Meganyctiphaunes norvegica were maintained in antibiotic-treated ("A"), and untreated (control, "C") aquaria. Each 24 h (for 72 h) numbers of saprophytic (Colony Forming Units, CFU), chitinolytic, and total bacteria (Acridine Orange Direct Count, AODC), were determined in the stomachs and hepatopancreata of animals. Parallely, the activities of chitinase, N-acetyl-β-d-glucosaminidase (NAGase), protease, cellulase and laminarinase were measured, and bacterial and endogenous chitinases compared by Fast Protein Liquid Chromatography (FPLC). CFU in "C" group stomachs were stable, but varied widely in the "A" group. AODC did not vary to any great extent. Hepatopancreas CFU decreased by four orders in the "A" compared to the "C" group. In "C" hepatopancreata, AODC were slighly higher than in "A". Numbers of chitinolytic bacteria fell steeply in the "A" group. Chitinase, NAGase, protease, and cellulase activities in the stomach were significantly lower (5% level) after 24 h in "A" than in "C". Laminarinase activity was significantly reduced in both organs after 72 h. Hepatopancreas chitinase and protease activities showed significant reductions only after 24 h. FPLC elution profiles of NAGase from krill, and bacteria grown on a medium semi-selective for chitinolytic bacteria differed. A number of bacterial chitinase peaks were absent in the "A" group. The API Zym system showed reduced proteolytic and lipase (C14) activity over 72 h. Endogenous bacteria are considered commensal with respect to the enzymes discussed. © 1995.