During 1975 and 1976 an FPS-18 radar with a 10.9-cm wavelength was used to study lightning in thunderclouds over Langmuir Laboratory in central New Mexico. The radar antenna beam was pointed at a thundercloud, and then held fixed for extended periods. With this system radar echoes from 156 lightning flashes were detected at altitudes ranging from 5 to 14 km above sea level. About one-half of the lightning echoes were detected at altitudes greater than 8 km at temperatures less then -20¿C. The lightning echoes were strong but transient with durations between 10 and 600 ms. The echo extents along the fixed radar beam ranged from 2000 m. Most of these echoes rose to peak intensity in less than the 1 ms pulse-repetition period of the radar and showed fine-structure fluctuations of the order of 5 to 10 dB with decay time constants of about 2ms. The durations of the sustained lightning echoes show no correlation with the final echo-decay times. The electric-field changes (measured at the Laboratory) that were associated with the lightning echoes observed at the higher altitudes ofter show a final slow, continuing variation that suggests that continued ionization may extend the echo-decay time. The thunder produced by the high-altitude discharges was weak and often was not detected from lightning echoes originating at altitudes greater than 9 km. We infer that the in-cloud portion of a lightning discharge is a tortuous set of small-diameter channel elements; than ion-recombination times are in the order of milliseconds; and that channel temperatures of about 3000¿K and electron densities of about 1016 m-1 are often maintained for the duration of the lightning echo by a continuing discharge process.