Potato seedlings were evaluated at 2, 3, and 4 wk of age for resistance to Empoasca fabae (Harris) using glasshouse methods. The relative resistance rankings of seedling-stage plants, as measured by nymphal survival and plant injury, were similar to those obtained by field assessment of mature plants in earlier studies. Resistance of the wild, diploid species, Solanum pampasense Hawkes, was detectable in seedlings as young as 2 wk of age. Depending on the growth stage, foliage of S. pampasense seedlings contained 3–14 times the levels of total glycoalkaloids as that of the susceptible cultivar, ‘Katahdin’, suggesting involvement of these steroidal glycosides in potato leafhopper resistance. For mature plants, use of a fluctuating 25°/l5°C temperature regime provided a wider range in expression of resistance between clones than did a constant 25°C regime.