We use observations from a high-frequency Doppler radar experiment at midlatitudes to study the wavelength dependence of spectrum broadening of backscatter from field-aligned E region irregularities. The radar made multifrequency measurements of high-resolution Doppler spectra simultaneously at four frequencies, that is, 9.23, 11.03, 12.71, and 16.09 MHz, which correspond to backscatter from irregularities with wavelengths of 16.3, 13.6, 11.8, and 9.3 m, respectively. In the analysis we deal with secondary irregularities (type 2 echoes) characterized by small mean velocities that show no dependence on radar frequency and have mean spectrum width to velocity ratios well above unity. The spectrum width is found to increase monotonically with wavenumber k in the range from 0.38 to 0.67 m-1 covered in the experiment. By postulating that the width is determined mainly by the nonlinear growth rate of the secondary short-scale plasma turbulence, we compare the results to the Sudan [1983> theory. Although there is some general agreement, on the average, the measured mean spectrum width follows approximately a k4/3 power law dependence which is considerably stronger than the theoretical k2/3 dependence. This discrepancy may be reduced somewhat but cannot be fully accounted for when additional spectrum broadening effects caused by the irregularity velocity distribution are also included. ¿ 1998 American Geophysical Union