A total of 479 calves in 27 herds were examined for diarrhoea and enteric pathogens. The overall prevalence of diarrhoea was 13.6%. The characteristics of diarrhoea faeces viz, the consistency, colour and smell were evaluated and related to the causative agents. A loose greenish nonsmelly diarrhoea was suggestive of helminthic infections while whitish, bloody or mucoid diarrhoea was suggestive of coccidial and bacterial diarrhoea. The proportion of calves which survived for 12 0 days without diarrhoea was 91.3%. The probability of calves having diarrhoea was high among age-group 0-30 days. This indicated the high risk period. There was no significant difference between the occurrence of diarrhoea and either geographical locations of herd, year or season. Odds ratios indicated associations between some enteric pathogens and diarrhoea. These enteric pathogens were E. Coli + Toxocara sp; E. coli + Coccidia; E. coli + Strongyloides sp. (in older calves); and E. coli + Toxocara sp. + Coccidia. Important pathogens responsible for diarrhoea in the first 30 days of life were E. coli + Coccidia; E, coli + Toxaocara sp; and E. coli + Toxocara sp. + Coccidia. It is suggested that E. coli may be an important factor to be present for gut parasites to cause diarrhoea. The presence of E. coli enterotoxins were not related with the occurrence of diarrhoea. Thus if E. coli caused the diarrhoea observed, it might have done it by the Abstract vii presence of other factors other than enterotoxins in E. coli responsible for diarrhoea. One of these factors was the ability of E. coli to haemagglutinate Ox RBC. E, coli strains isolated from 65 and 41 diarrhoeic and nondiarrhoeic calves respectively were characterized. In this work, serogroup 08 was among the commonest 0 serogroups identified. Mixed infections of enterotoxigenic E. coli (ETEC) and gastrointestinal parasites (GP) caused 11.8% of the diarrhoea, while mixed infection of E. coli strains which agglutinated Ox RBC caused 9.8% of the diarrhoea. While the combination of GP and E.coli strains contributed 23.5%; and E. coli strains alone contributed 5.9% of the diarrhoea. The freguencies of enterotoxins identified were ST (8.1%), and LT, (3.3%) from diarrhoeic calves while the freguencies of verocytotoxins were 3.3% and 9.8% from diarrhoeic and non-diarrhoeic calves respectively. The property of E. coli to haemagglutinate Ox RBC (16.4%) was of significant finding in diarrhoeic calves. A total of 17 lambs were used for experimental reproduction of enteric colibacillosis. The lambs were divided into 3 groups of 3, 4, and 10 lambs each. The first 3 lambs were in group one which served as blank control lambs that were administered 5.0ml of sterile Brain Heart Infusion Broth; 4 lambs in group two were positive control lambs that were administered 5.0ml of Brain Heart Infusion Broth containing about 3 x 108 E. coli organism/ml, while 10 lambs in group three were experimental lambs that were Abstract viii administered 5.0ml of Brain Heart Infusion broth containing about 3 x 108 E. coli organisms/ml. The average pack cell volume, protein levels, temperature (°C), pulse and respiratory rates were recorded before and after infection. From group one, lamb 01 developed mild non-smelly pasty diarrhoea probably due to change in feed. From group two, lambs 04 and 07 had severe smelly and pasty diarrhoea and latter died (50% case fatality rate) suggesting that the E. coli strain used was enterotoxigenic in lambs. From group three 5 lambs developed severe smelly and pasty diarrhoea and case fatality rate was 80%. Experimental infection of lambs suggested that the E. coli strain from diarrhoeic calf in Zaria was enterotoxigenic in lambs. Cross infections can easily occur between lambs and calves kept together. The average values of temperature, pulse and respiratory rates were not significantly different between groups before or after infections suggesting non-involvement of circulatory system. In conclusion further studies are needed to evaluate the roles of viruses, parasites, bacteria and management practices on the prevalence of calf diarrhoea in Zaria. Experimental infections of calves with single and multiple combinations of common enteric pathogens is suggested to determine experimentally any correlation between the nature of diarrhoea and enteric pathogens. This information will be helpful in making a clinical diagnosis of diarrhoea in calves.