Investigation on avian pathogenic Escherichia coli (APEC) in clinically healthy and diseased commercial chickens was conducted between December 2012 and February 2013 in Arusha City, Tanzania. One-hundred seventy six (75.9%) E. coli isolates were obtained from 168 cloacal swabs from healthy chickens and 64 visceral organs of sick chickens were subjected to bacteriological culture, biochemical tests, antimicrobial susceptibility test and serotyping using O1, O26 and O78 E. coli antisera. The results showed that 81.0%; (n= 136) of cloacal swabs and 62.5% (n= 40) of visceral organs carried E. coli. Findings of serotyping showed that 65 (36.9%) were identified as pathogenic strains belonging to serotype O1 (30), O26 (6) and 078 (29) E. coli isolates. Whereas (63.1%, n=111) of the isolates were untypable with the panel of antisera used. Serotype O1 was significantly more prevalent in visceral organs than in the cloacal swabs (P<0.05). Antimicrobial susceptibility studies showed high resistance to Sulphamethoxazole (98.3%), Cotrimoxazole (97.7%) and Ampicillin (42.6%). Multiple resistance to two and more antimicrobials were observed and ranged between 0.6 and 43.2%.  High proportions of isolates were found to be susceptible to Gentamycin (97.2%), Streptomycin (85.9%) and Kanamycin (83.6%). The findings of APEC strains in the diseased chickens as compared to healthy flock suggest the involvement of these strains in the pathological process as a primary or secondary invader. However, further investigations are required to understand the link of pathogenic strains to other chicken diseases.  A high proportion of E. coli isolates was resistant, including multiple resistance to commonly used antimicrobial. Further investigations on E. coli by serotyping, molecular typing and antimicrobials susceptibility studies are recommended.