Aim: This study identified and enumerated microorganisms associated with the composting of some organic wastes using the plate count method Study Design: The wastes were allowed to decompose for 70 days in greenhouse using the modified windrow method of composting. Standard methods were employed to monitor temperature changes in compost piles as well as changes in bacterial and fungal populations. Place and Duration of Study: This study was carried out at in the Agricultural Research Centre of the Federal University of Technology, Owerri, Nigeria. Methodology: The organic wastes  namely Poultry Litter (PL), Pig waste (PW), Cow dung (CD) and Source-Separated Municipal Solid Waste (MSW) were composted/co-composted using the windrow method as modified .Sixty kilograms (60) each of PW, PL, CD and MSW were introduced respectively into 100-litre(L) buckets that had previously been perforated at several points. In the co-composted piles, 30 kg of both samples were introduced into the same 100 L bucket that had previously been perforated and then mixed thoroughly. The organic wastes were allowed to decompose at room temperature at a corner of the greenhouse. and initial microbial populations as well as subsequent populations  in the compost bins were studied using standard microbiological methods Results: Microbial populations increased concurrently with temperature during the first 3 – 4 weeks of composting except, however, for faecal coliforms and Salmonella. The highest temperature recorded was 60°C for cow dung (CD) compost pile though at maturity the temperature in all compost piles ranged between 27°C to 30°C. The bacterial colony forming units were higher than fungal colony forming units throughout the composting period for both mesopholic and themophilic microorganisms. The population of mesophilic organisms increased in the first 14 – 15 days; for cow dung, the initial total heterotrophic bacteria count (THBC) and total  coliform count (TCC) were 2.4 x107 cfu/g and 5.0 x105 cfu/g respectively and  increased to 2.5x108 cfu/g  and 1.7x107 cfu/g  for THBC and TCC, respectively, after the 14th day. Thermophilic bacteria dominated all the composting systems after the 21st day and lasted to the 35th day except for cow dung compost where thermophilic temperatures were still observed on the 45th day with a THBC of 6.3x106 cfu/g on the 49th day. Faecal coliforms and Salmonella were completely eliminated in all the compost systems after the 28th day with temperature values between 47°C – 60°C. Conclusion: Organic wastes when managed properly through the application of knowledge of composting can be transformed into beneficial materials for human and agricultural use.