IN the period between September 15 and October 14, 1965, the radio source 3C273, observed with the east-west arm of the “Northern Cross” at 408 Mc/s, showed interplanetary scintillation. Comparison of the three records obtained with the three independent beams of the instrument excludes spurious effects due to instability or interference. The distance of the line of sight from the Sun during the period investigated ranged between 0.3 and 0.08 astronomical units. We observed a continuous increase in scintillation as the distance decreased. Taking into account the limit stated by Hewish1, extrapolated to our frequency, the diameter of the core of the B component of the source 3C273 should be equal to or less than 0.1 sec of arc. This is in contrast to the value of 0.5 sec of arc calculated by Scheuer2 from the record of the lunar occultation observed by Hazard3. This discrepancy is explained by the recent discovery of the composite structure of the B component4.