The technique of the symmetric double probe lends itself to thein situmeasurement of plasma temperature in the ionospheric D region because it can lead to meaningful results under relatively high collision frequencies where the Langmuir probe has been observed to fail. For a completely thermalized plasma and over parametric ranges 0.1≤&lgr;ia/Rp≤100 and 0.004≤Rp/&lgr;D≤100, where &lgr;iais the ion‐atom mean free path, Rpis the probe radius, and &lgr;Dis the Debye length, it is shown that the modification to the original collision‐free double‐probe theory of Johnson and Malter for the determination of electron temperature is never greater than 12%, with a value of (8 ± 2)% nominally applicable in the case of D‐region diagnostics. This technique has been successfully operated on a Nike‐Cajun payload flown at midday from White Sands, N. M. to an apogee of 78.5 km. The associated electronics and deployed double‐probe configuration are presented, and a current‐voltage characteristic collected in the ascent stage at 73.7 km is briefly discussed. The values of electron temperature indicated by the sampled data are [inverted lazy s] 30% higher than those predicted by theory for the anticipated state of thermal equilibrium with the ambient neutrals. The importance, however, is that these values, of the order of 300 K, are the lowest ever reported by anyin situmethod operative in the D region and consequently point out the strong potential of the double‐probe technique in future applications to regions of planetary ionospheres which cannot be considered as collision‐free.