A non-thermal influence of millimeter wave radiation (swept in frequency from 64.1 GHz to 69.1 GHz, sweeptime 6 s, and with stabilized frequencies of 67.200 +/- 0.001 GHz and 68.200 +/- 0.001 GHz, power density less than or equal to 6 mW/cm2) on the puffing of giant chromosomes of the midge Acricotopus lucidus (Diptera, Chironomidae) was found. The effect is manifested as a reduction in size of a specific puff that expresses genes for a secretory protein. The non-thermal nature of the effect was proved by experiments in which the sham-exposed sample was warmed up by 2.5 degrees C which is more than the eight-fold microwave induced temperature increase of less than or equal to 0.3 degrees C. Concerning the very low photon energy of mm-waves compared to the thermal energy kT, it seems likely that the coherence of the radiation is essential for the observed effect.