Bright-floored and dark-floored craters on Venus show systematic differences in their size, distribution and apparent modification. Bright-floored craters exhibit the following characteristics: an interior radar brightness comparable to the youngest craters on Venus, a tendency toward smaller crater diameters, and a broad range of crater elevations. In contrast, the dark-floored craters are darker than pristine craters on Venus, are typically much larger, and preferentially occur at lower elevations. They also have larger floors than pristine craters of the same size and are similar in many respects to floor-fractured craters on Venus. Of the four proposed origins for dark crater floors, these observations are most consistent with crater-centered volcanism. Surface weathering or eolian deposition may contribute to floor darkening in some cases, but neither of these mechanisms, nor impact melts, can independently explain the full range of observed modifications. Based on the measurements of this study, volcanism has affected at least 6--13% of the impact craters on Venus. The estimated lava fill thicknesses are of the order of 100--500 m. Thus, if these flows are representative of eruptions elsewhere in the Venusian lowlands, the implied average flux of plains-forming volcanism on Venus for the last 300 m.y. is ~0.01--0.02 km3/yr. ¿ 1999 American Geophysical Union