Summary. From December 1989 to May 1990,3 15 faecal samples from children under 5 years old with diarrhoea (215) and without diarrhoea (100) seen at paediatric clinics were investigated for bacterial, viral and parasitic enteropathogens. Standard and recently described methods were used for the investigations, which revealed that 74.9 YO of children with diarrhoea were infected with enteropathogens compared with 28 YO of controls. In the diarrhoeal group, 59.1 YO had a bacterial, 26.5 O/ O a viral and 2.3 YO a parasitic aetiology. Rotavirus was the pathogen most frequently detected, accounting for 22.3 YO of positive findings in the group with diarrhoea versus 9 YO in the control group. Other important agents were : enterotoxigenic Escherichia coli (ETEC) (14.4 versus 6 YO), enteropathogenic E. coli (EPEC) (10-7 versus 5 YO), enteroadherent E.coli (EAEC) (9.3 versus 4 YO), enterohaemorrhagic E. coli (EHEC) (5.1 versus 3 YO) and Salmonella spp. (3.3 versus 1 YO). The following enteropathogens were detected exclusively in the diarrhoeal stools : Shigella spp. (5.1 YO), Yersinia enterocolitica (0-9 YO), Aeromonas hydrophila (1.4 YO), Entamoeba histolytica (0.5 YO), Giardia lamblia (0.5 YO), Trichomonas hominis (0-5) and Trichuris trichiura (0.9 YO). The detection rates of rotavirus, EPEC and EAEC were much greater in the diarrhoeal than in the control patients. No Vibrio cholerae, enteroinvasive E. coli (EIEC), Plesiomonas spp. or Cryptosporidum spp. were detected in this study. Our data suggest that both the traditional and newly recognised diarrhoeal agents are important causes of diarrhoea in the children under 5 years old in Lagos, Nigeria.