Particulate matter (PM) with an aerodynamic diameter of <2.5 micrometers (PM2.5) increases the risk of cardiopulmonary and lung cancer mortality and morbidity. Bangladesh is one of the many developing countries in Asia facing severe PM2.5 air pollution problems. In this study, chemical composition and size distribution of ambient PM2.5 in Bangladesh have been characterized in the megacity of Dhaka and for the first time in a background location of Noakhali. PM2.5 size distribution and mass concentrations were measured at a traffic hotspot and a residential area in Dhaka and in a residential area in Noakhali during the rainy monsoon season and dry winter season in the year 2009/2010 using two MOUDI samplers, two SKC gravimetric samplers, two TSI DustTraks, and one TSI P-Trak. Gravimetric filters were analyzed for mass of PM2.5, elemental carbon (EC), organic matter (OM), and 42 elements by X-Ray Fluorescence (XRF). During the winter season mean 24- hour (24-hr) PM2.5 concentrations in Dhaka were 322 ± 69 µg/m3 and 254 ± 57 µg/m3 in the traffic hotspot and residential area, respectively. During the monsoon season mean 24-hr PM2.5 concentration in the Dhaka residential site was 57 ± 15 µg/m3. Of the PM2.5 in Dhaka, approximately 42-56% is OM and 8-22% is EC in nature. These results exceed the 24-hr WHO guideline of 25 _g/m3 and EPA standard of 35 µg/m3. Mean 24-hr PM2.5 concentrations in the Noakahli residential site were 65 ± 9 µg/m3 during the winter season and 21 ± 9 µg/m3 during the monsoon season, suggestive of lower PM2.5 concentrations than in Dhaka. However, it still experiences prominent PM2.5 particularly in the wintertime. Of the PM2.5 in Noakhali, approximately 11% is OM and 5% is EC in nature. PM1.0 (PM <1.0 _m) concentrations were recorded for the first time in Bangladesh, with an average of 34,596 ± 7,161 particles/cubic centimeter (pt/cc) at the Dhaka traffic hotspot in the winter season. In the same site and season, size distribution of PM1.8 was also performed for the first time in Bangladesh, with an average concentration of ultrafine particles (UFPs), PM <0.1 _m in diameter, of approximately 4 µg/m3. PM1.8 size distribution showed a dominant size range between 1.0 to 1.8 _m in diameter, yet concentrations of PM1.8 between 0.02 to 1.0 _m were also present. Chemical species such as chlorine (Cl), potassium (K), lead (Pb), sulfur (S) and silicon (Si) were reported in significant amounts in Dhaka with mean concentrations of 4.7, 6.1, 0.6, 5.9 and 13.7 _g/m3, respectively. Local sources and long range transport of pollutant may be a factor leading to elevated PM levels at the traffic hotspot and residential site in Dhaka as well as in the background residential site of Noakhali. Results from this study propose the need for air quality control in the rapidly growing urban city of Dhaka to improve the cardiopulmonary and respiratory health of individuals affected by ambient air pollution in Bangladesh.