The Cornell University Portable Radar Interferometer (CUPRI), a 50-MHz Doppler radar system, was operated during May and August/September 1983 on the island of St. Croix (17.7¿N, 64.8¿W) to study the plasma instabilities associated with nighttime sporadic E layers. Two events, on May 7 and August 22, show evidence of large-amplitude waves, with apparent horizontal wavelengths of 10-12 km and periods of 2-6 min. These apparent wavelengths are upper limits for the true wavelengths. The CUPRI beam was directed over Arecibo, Puerto Rico, and on May 7, concurrent electron density profiles within the CUPRI scattering volume were measured by the Arecibo Observatory's 430-MHz radar. During the stronger event of August 22, radar echoes were received from several altitudes up to 130 km. Large mean Doppler velocities (at times exceeding 250 m/s) were observed during this event, and the power spectra closely resemble those obtained at the magnetic equator during type 1 conditions. We believe (1) that the mid-latitude large-scale waves are generated by the same gradient drift instability mechanism responsible for equatorial large-scale waves and (2) that the type 1 3-m waves can be generated at mid-latitudes with drift velocities well below the sound speed because of the very sharp plasma density gradients associated with metallic ion sporadic E layers.