Detailed measurements of electric field wavevectors of the Farley-Buneman two-stream instability in the daytime equatorial electrojet were gathered on a sounding rocket flown from Alca⁁ntara, Brazil. Primary two-stream waves were detected in the altitude region of 104--108 km, where the current density and polarization electric field maximized and where intense 3 m vertical echoes were observed by a 50 MHz backscatter radar. The in-situ measured wave components are predominantly oriented in the horizontal, zonal direction. Spaced receivers reveal wavelengths of 11--14 m with phase velocities of about 370 m/s at the peak wave power near 106.5 km. These velocities are constant from 1--15 m, indicative of dispersionless flow. Although the neutral wind was not measured, the in-situ phase velocity appears to be below that predicted by linear theory.¿ 1997 American Geophysical Union