The amplitude of the 54 Mc/s signal from the satellite Transit 4A was recorded at two stations, 100 km apart, from March to August 1964. The time difference between observing similar fluctuations in the fading period at the two stations was used to determine the height of ionospheric irregularities. 44 irregularities were observed, predominantly between the hours 0800 and 1800 local time, at heights from 180 to 750 km. The horizontal sizes varied from 75 to 520 km. The denser irregularities occurred only near the peak of the ionosphere, between 250 and 400 km, so that the mean percentage density fluctuations were approximately the same at all heights. Two examples of trains of irregularities were found, in which the irregularities lay on a straight line tilted down to the south at an angle of about 15 degrees. There was some evidence that the individual irregularities were also tilted down to the south by this amount.