Alfalfa cultivars having the glandular-haired trait and selected for improved resistance to potato leafhopper (Empoasca fabae, Harris) were released commercially beginning in 1997. Limited information is available on the population dynamics of potato leafhopper in these glandular-haired alfalfa cultivars under natural field infestations. The objective of this research was to monitor potato leafhopper populations in glandular-haired and standard susceptible alfalfa cultivars in both small (1.5 x 7.6 m) and large (12 x 20 m and 30 x 38 m) replicated field plots. Commercially released and advanced experimental lines were monitored. Sweepnet and pan trap sampling techniques were used to sample potato leafhopper on a weekly to twice-weekly interval over three years and two locations. Potato leafhopper densities reached high levels each year, depending on the location. Significant differences in potato leafhopper populations were found among cultivars on many sampling dates. In a 1997-seeded small plot experiment in northwest Ohio, the maximum potato leafhopper density in the susceptible check cultivar was 15, 5, and 11 leafhoppers per sweep in 1997, 1998, and 1999, respectively. In contrast, the leafhopper density in the advanced experimental lines was 30 to 40% of that in the susceptible cultivar. In a 1999-seeded small plot experiment in northeast Ohio, leafhopper density reached a maximum of 29 to 32 per sweep in the susceptible check cultivars, but only 6 to 12 leafhoppers per sweep in advanced experimental lines. The trend for lower leafhopper densities in the experimental lines was consistent across all sampling dates, even when leafhopper densities were relatively low. These experimental lines, developed by private breeding programs, had undergone selection to improve the level of resistance to potato leafhopper. Results from the large plot studies demonstrated similar reductions in leafhopper populations in glandular-haired cultivars (DK121HG and DK131HG) as compared with standard susceptible cultivars (DK121 and DK134). In 1997, maximum density reached 30 leafhoppers per sweep in DK121, whereas only 9 leafhoppers per sweep were found in DK121HG on the same date. In 1998, maximum density in DK121 was 13 leafhoppers per sweep compared with only 6 per sweep in DK121HG. Maximum density in DK134 in 1998 was 10 leafhoppers per sweep, whereas in DK131HG it was 5 leafhoppers per sweep. Similar large-plot studies on farms around Ohio have confirmed these findings. In these and other experiments not reported here, the level of reduction in leafhopper density in glandular-haired cultivars has often not been sufficient to eliminate economic yield losses; however, the loss has been significantly less in glandular-haired cultivars as compared with standard susceptible cultivars. Both the small and large plot studies demonstrate that glandular-haired alfalfa appears to have a negative impact on adult leafhopper numbers. This is followed by a large negative impact on nymphal populations in glandular-haired alfalfa compared with standard alfalfa cultivars. The exact mechanism for lower nymphal populations in glandular-haired alfalfa cannot be determined from these studies; however, based on the differences in adult populations, we conclude that antixenosis (formerly referred to as nonpreference) appears to play a role in the leafhopper resistance of glandular-haired alfalfa under natural infestations in small and large field plot studies.