Ionospheric data over Ahmedabad (23 ¿N, 73 ¿E) during 1955--96 have been examined for the long term changes arising due to the predicted cooling of the mesosphere and thermosphere because of the increased concentration of trace gases resulting from anthropogenic causes. The long term changes in the critical frequencies of the ionospheric layers and the height of the maximum ionization as characterized by hPF2, the virtual height at 0.834 of foF2, have been obtained after removing the seasonal and solar cycle variations. While the trend of a small decrease in foE (0.15 MHz in 40 years) may not be considered significant, there is a clear indication of a decrease in foF2 (1.6 MHz for midday, 1.0 MHz for midnight). A small increase is noted in foF1 (0.3 MHz). There is a decrease in hPF2 (16 km for midday and 10 km for midnight). The lowering of F2 layer peak will affect radio propagation. ¿ 1999 American Geophysical Union