Bio-monitoring of air quality in Amman City was investigated by analyzing 36 cypress tree (Cupressus semervirens L.) bark samples from three sites of different anthropogenic activities at the end of summer season 2001. Cypress barks were found to be a good bio-indicator for air pollution in arid regions. Variation in Pb, Zn, Mn, Cr, Ni, Cd, and Cu contents between sites was observed due to different types of activities. Traffic emissions were found to be the main source of heavy metal pollution in the atmosphere of Amman. Lead content was found to be the highest in highly traffic density areas. The industrial part of the city was characterized by high Zn, Mn, Cr, Ni, and Co contents. No significance variations were found in pH values of the bark between the sites. This was attributed to buffering effect of carbonate in the atmosphere originated from soil of the area.