Alnus maritima (Marsh.) Muhl. eh Nutt. (seaside alder) forms root nodules in which atmospheric nitrogen (N) is fixed by bacteria in the genus Frankia Brunchorst. Plants in nurseries are fertilized with N and exposed to high root-zone temperatures; both of these factors may influence nodulation of A. maritima. Our objectives were to determine how applied N and root-zone temperature affect nodule formation and function. For all experiments, plants of Alnus maritima subsp. maritima were inoculated with soil containing Frankia from the Delmarva Peninsula. To assess N effects, plants were grown in perlite and treated for ten weeks with nutrient solutions that contained ammonium nitrate at 0, 0.25, 0.5, 0.75, 1 1.25, 1.5, 2, 4, and 8 mM. As applied N increased, nodule formation and activity decreased. Plants in the 0.5-mM treatment were robust and produced an average of 70 nodules, while plants treated with ammonium nitrate at 8 mM formed no nodules. Using this information, we provided 0.5-mM ammonium nitrate to additional plants for six weeks to produce a healthy, well-nodulated population of A. maritima. Those plants were then treated with ammonium nitrate at 0.5, 2, or 4 mM for two weeks followed by daily application of N-free solution. Nodule activity of plants treated with 4-mM ammonium nitrate was suppressed. Daily irrigation with N-free solution leached excess N from the root zone and led to a resurgence in nodule activity. To assess temperature effects, we grew seedlings with roots at 20, 28, and 360C for six weeks. The greatest growth and nodule activity occurred among plants with roots at 280C. Next, prenodulated alders were treated with 420C for 0, 6, 12, and 24 h·day−1 for one week. Nodules on root systems exposed daily to 420C for 6, 12, and 24 h·day−1 fixed N at similar rates, whereas plants treated for 0 h·day−1 (24 h·day−1 at 280C) fixed the most N on the basis of nodule dry weight. These data provide information that will be useful to those concerned with growing well-nodulated plants of A. maritima.