A study of prevalence of mycoplasmosis of chicken at kotwali thana in Chittagong was conducted from 23rd November, 2010 to 22nd March, 2011. A total of 455 either dead or sick birds were examined during the period. Diagnosis was based on history, clinical signs, serum plate agglutination (SPA) test and postmortem lesions. The overall prevalence of mycoplasmosis was 15.38%. The prevalence was 73% in broiler and 27% in layer. The result further revealed that highest number of cases were recorded in the age group of 8-21 days (49%) followed by 22-35 days age group (21%), 0-7 days age group (21%), 36-60 days age group (6%) and over 60 days age group (3%). It was also demonstrated that during the study period the highest prevalence of mycoplasmosis was recorded in January (33%) followed by February (30%), March (23%), and December (13%) and lowest in November (1%).