The present study describes the clinical, serological and bacteriolological findings in calves from two beef herds experiencing outbreaks of pneumonia. The clinical signs were nasal discharge, cough, pyrexia and increased respiratory rates. The morbidity and mortality rates over a month period were 40.72% and 15.63% respectively. Laboratory investigations revealed that bovine viral diarrhea virus (BVDV) was involved in and probably initiated both outbreaks as indicated by a significant increase in antibody titers against BVDV in sera of convalescent calves (paired serum samples). No antibodies bovine herpesvirus-1 (BHV-1), bovine respiratory syncytial virus (BRSV) and parainfluenza-3 (BPIV-3) viruses were detected in both acute and convalescent sera. Mycoplasma bovis was concurrently demonstrated in lungs of affected calves as it was isolated from 13 (81.25%) of examined lungs suggesting that there may be a synergism between bovine viral diarrhea virus and Mycoplasma bovis in the pathogenesis of pneumonia. A total of 15 (68.18%) isolates of Mannheimia haemolytica, 5 (22.73%) Pasteurella multocida, 1 (4.54%) Pseudomonase aerugenosa, 3 (13.64%) Staphylococcus aureus, 3 (13.64%) Actinomycis pyogenes, 1 (4.54%) Klebsiella pneumonae, 1 (4.54%) Streptococcus pneumonae, 2 (9.09%) E. coli and 2 (9.09%) Aspergellus fumigatus were recovered from lungs of calves suffering from pneumonia.