The effects of ionospheric heating on low-power diagnostic radio waves at Arecibo are reported. During O mode heating it was found that the signal strengths of the diagnostic waves were reduced by up to approximately 7 dB. The diagnostic signals are only significantly attenuated during heating at full power (effective radiated power of 80 MW). During X mode heating, no amplitude effects could be detected. The maximum attenuation of the diagnostic signals measured at Arecibo is lower than those typically observed at Troms¿, which is consistent with the predictions of theory. These results are discussed in the context of theoretical calculations of the anomalous absorption due to mode coupling caused by small-scale field-aligned irregularities. From observations of the effects of heating on the amplitudes of several low-power diagnostics, the length of these irregularities along the geomagnetic field is estimated. ¿ American Geophysical Union 1992