The purpose of this study was to determine the concentrations of fine particulate matter (PM2.5) at six subway stations. The PM2.5 levels were compared at the platforms, inside the trains' cabin and at the train driver's cabin in the first section of the train. The measurements were grouped into three sections: (1) six underground platforms, (2) trains' cabin and (3) train driver's cabin in the Aksaray-Airport light metro line (M1) and the Taksim-4. Levent metro line (M2). On average, the highest PM2.5 concentrations were recorded in the underground platforms. The average daytime PM2.5 concentrations varied from 49.3 to 181.7 mu mg.m(-3) at the two subway line platforms. The PM2.5 levels measured at five stations were higher than the ambient air PM2.5 standard declared by US-EPA, given as 35 mu g.m(-3) for a 24-h arithmetic mean. The second highest PM2.5 concentration was recorded inside the trains (61 to 73 mu g.m(-3)), followed by train driver's cabin (50 to 74 mu g.m(-3)) of the M1 and M2 lines. When the train was in the subway tunnels, the PM2.5 concentrations increased. The correlation (R-2) between the PM2.5 level on the platform and the depth of the underground stations was 0.88 (p<0.01). The highest PM2.5 levels were recorded in the morning and evenings.