An experiment to measure the electric field E and dE/dt signatures that are radiated by the first return stroke in cloud-to-ground lightning was conducted on the eastern tip of Cape Canaveral, Florida, during the summer of 1984. At this site, there was minimal distortion in the fields due to ground wave propagation when the lightning struck within a few tens of kilometers to the east over the Atlantic Ocean. Biases that are introduced by a finite threshold in the triggered recording system were kept to a minimum by triggering this system on the output of a wideband RF receiver tuned to 5 MHz. Values of the peak dE/dt during the initial onset of 63 first strokes were found to be normally distributed with a mean and standard deviation of 39¿11 V m-1 μs-1 after they were normalized to a range of 100 km using an inverse distance relation. Values of the full width at half maximum (FWHM) of the initial half-cycle of dE/dt in 61 first strokes had a mean and standard deviation of 100¿20 ns and were approximately Gaussian. When these results are interpreted using the simple transmission line model, after correcting for the effects of propagation over 35 km of seawater, the average value of the maximum current derivative, (dI/dt)p, and its standard deviation are inferred to be 115¿32 kA μs-1, with a systematic uncertainty of about 30%. The FWHM after correction for propagation is about 75¿15 ns. The inferred values of (dI/dt)p are significantly higher than most previous measurements of natural first strokes during direct strikes to instrumented towers but are in good agreement with direct measurements of dI/dt during subsequent return strokes in rocket-triggered discharges in Florida. ¿ American Geophysical Union 1996