An investigation of the horizontal structure of sporadic-E layers has been made using the incoherent scatter radar at the Arecibo Observatory. Data are presented for two observations, one with the radar beam held in a vertical position, and another with the beam scanning in azimuth across the sporadic-E layer. The first observation was made at sunset and shows the passage of a large region of ionization, about 150 km in extent, having little small-scale horizontal structure. The second, at midday, shows considerable variation in the value of the maximum electron density in the vertical cross section of an intense sporadic-E layer. The horizontal dimensions of the features range in size down to the resolution of the radar (300 m). By considering the data statistically it is shown that small patches, 300 m or less in horizontal extent, exist in the sporadic-E layer with densities great enough to account for the maximum frequency of the echo recorded on the ionosonde located at the observatory.