Knowledge of potential breeding density and associated reproduction is critical for conserving Piping Plovers (Charadrius melodus) in the northern Great Plains. Each year during 1995–1998 we observed 34–50 plover pairs nesting on Appam Lake, an isolated 1.9-km² alkali lake in northwestern North Dakota. During peak abundance in 1996 and 1997, there were 4.2 and 5.3 pairs/km of available beach, respectively; mean distances between nest sites of nearest neighbor pairs were 99.4 m and 79.5 m. We detected no relationship between spacing and numbers of fledglings produced by individual breeding pairs. Mean production was 1.2 fledglings/pair (0.9 during 1996 and 1.5 during 1997, n = 25 and 29 pairs, respectively). Our observations suggest Piping Plovers may reproduce adequately at high breeding densities on prairie alkali lakes, but further study of the relationship between breeding density and reproductive success is needed.