Significant differences have been proved between strains of E. coli isolated from neonatal E. coli diarrhoea (N.C.D.) and post weaning diarrhoea (P.W.D.). The biochemical differences were pronounced in the criteria sucrose, dulcitol, adonitol and urease when comparing the total number of strains from the two syndromes. However, adonitol positive strains were only found among strains of the serogroup O 149:K91(B) of which all the N.C.D.. strains were positive and only 8 of the 86 P.W.D. strains were adonitol fermenters. The dominant fermentation pattern of the N.C.D. strains were sucrose--, dulcitol--, urease--. Contrary the P.W.D. strains were sucrose +, dulcitol +, and 56.6 per cent urease + (Table III). In the possible plasmid determined characters significant differences were found in the ability to produce the K88 antigen and the colicins. The K88 antigen was demonstrated in 97.9 per cent of the N.C.D. strains and 6.2 per cent of the P.W.D. strains whereas 40.6 and 79.6% respectively were colicinogenic. In the serogroup O 149:K91(B) 90.7% of the P.W.D. strains and 47.6% of the N.C.D. strains produced colicins (Table V). The impairment of these features in the pathogenesis of neonatal E. coli diarrhoea, post weaning diarrhoea and the possible causality of the domination of the serogroup O 149:K91(B) is discussed. Furthermore it seems justified to relate the syndrome designation to the serotype of porcine enteropathogenic E. coli strains as it will be difficult to compare strains described from different research workers.