Microalgae are microorganisms with unique features of carbon sequestration and can act as biological indicators for air pollutants. Microalgae have been found to flourish in polluted environments. This study investigated the effect of carbon monoxide (CO) levels on the density of microalgal cells in different locations in Peninsular Malaysia, which has tropical climates. The species composition of microalgae influenced by different CO levels was also investigated. Microalgae samples were collected from four selected study sites, chosen based on average CO readings for 2015 in different categories which are low (Jerantut), medium (Banting, Kuala Lumpur), and high (Petaling Jaya) levels of CO. The findings of this study demonstrated a positive correlation between the CO concentration and the number of microalgal cells (Figure 1). This study also discovered that Desmococcus olivaceus is the most prevalent species found in the study areas and Trebouxia arboricola is the most dominant species. Trentepohlia rigidula, Coccomyxa confluens, Keratococcus bicaudatus, and Klebsormidium flaccidum can be classified as non-tolerant to high concentrations of CO as they are only found in areas with low concentrations of CO. Data from the Jaccard Similarity Index indicates a low similarity in species composition between Jerantut and; Banting, Kuala Lumpur, and Petaling Jaya with Jaccard coefficients of 0.2857, 0.1250, and 0.1250, respectively. The outcome of this study is important to understand the relationship between air pollutants and microalgae. Species distribution of microalgae suggests the use of epiphytic terrestrial microalgae as ecological indicators for air pollution. In addition, the species composition of microalgae influenced by air pollutants serves as a consortium for efficient bioremediation by utilizing microalgae species with high tolerance towards air pollutants.