  We observe a correlation between the slope of radio lateral distributions,
and the mean muon pseudorapidity of 59 individual cosmic-ray-air-shower events.
The radio lateral distributions are measured with LOPES, a digital radio
interferometer co-located with the multi-detector-air-shower array
KASCADE-Grande, which includes a muon-tracking detector. The result proves
experimentally that radio measurements are sensitive to the longitudinal
development of cosmic-ray air-showers. This is one of the main prerequisites
for using radio arrays for ultra-high-energy particle physics and astrophysics.
