A comparative analysis of culturable air borne microorganisms in post-monsoon and monsoon season was done by measuring their ambient levels at four different sites within Delhi premises which included both indoor and outdoor. Higher fungal concentration (between the range 36–97.33 CFU× 102 m-3) was found in post–monsoon season with Library canteen showing the maximum of 97.33 CFU× 102 m-3. A pattern of minimum concentration of fungal bioaerosol ranging between 2–6.6 CFU× 102 m-3 was seen in the monsoon period where Garbage site had the least concentration of 2 CFU× 102 m-3. Similar to fungal concentration, both gram positive and gram negative bacteria revealed maximum concentration in post monsoon season in comparison to monsoon season. Among all four sites maximum gram positive bacterial concentration of 93.33 CFU× 102 m-3 was found in library canteen in post-monsoon period while minimum concentration of 4.66 CFU× 102 m-3 in garbage site in monsoon period. The comparative analysis reveals higher concentration of viable air borne microbes in post-monsoon season than in the monsoon period at the indoor sites in comparison to the outdoor ones which may have resulted due to seasonal rain wash. Most of the fungal bioaerosol identified is associated with allergic and immunotoxic diseases such as sick building syndrome. Since the sampling occurred in two different seasons in outdoor and indoor sites with naturally ventilated buildings, external temperature and humidity did vary significantly during the study. Thus, it is more likely that the types of organisms and concentration level observed were related to both the sources (decomposing garbage, spores carried from wooden areas, etc.) as well as the ambient or indoor environmental conditions.