This is a preliminary report on one aspect of the radio propagation experiment on Pioneer 6. From an analysis of a small fraction of the differential group-path data from Pioneer and simultaneous ionospheric measurements using signals from two Beacon satellites, it has been determined that the average interplanetary electron number density near the earth's orbit was between 8 and 9 cm-3 in the time interval from February 20 to April 9, 1966. This is a spatial as well as a temporal average, since the measurements are of columnar electron content between the earth and spacecraft, corrected for near-earth (ionospheric and magnetospheric) electrons. During the indicated time interval, the spacecraft was between 10 and 31 million km from the earth and between about 0.93 and 0.86 AU from the sun. Temporal variations of the spatial-average number density caused an rms deviation from the mean of about 4.4 cm−3.