Cladosporium is a mycotoxin secreting and potentially pathogenic fungus frequently occurring in outdoor environments. In this study prevalence of Cladosporium species in the atmosphere within and around Riyadh city was monitored and production of mycotoxin by the isolates was assessed at different incubation temperatures under in vitro conditions. Two hundred air samples were collected from twenty locations of Riyadh. Only 20 samples were found to carry Cladosporium inoculum, belonging to two species, namely, C. cladosporioides and C. sphaerospermum. Crude extracts of the fungal cultures in acetone were scanned by spectrophotometry for presence of mycotoxins. It was noticed that cultures grown at lower temperature (10 and 15°C) yielded higher amount of mycotoxins as compared to cultures incubated at higher temperatures (20-30°C). HPLC assays of the extracts revealed five compounds corroborating with spectrophotometry findings of higher levels at low temperature. LC/GC-MS analysis revealed several compounds known for diverse activities.