Technical Abstract: The native perennial grass, Achnatherum robustum ([Vasey] = S. robusta [Vasey], Barkworth 1993), or sleepygrass has long been known as highly toxic and narcotic to livestock. The causative agent of toxicity is now known to be alkaloidal mycotoxins produced from infections by a systemic and asexual Neotyphodium endophyte. Early reports implied the toxicity was widespread but more recent studies suggest toxicity is limited across the range of sleepygrass in the Southwest USA. It is unknown how alkaloid levels vary among populations of sleepygrass, and within and among growing seasons, or for other native grasses infected with Neotyphodium. We sampled 17 populations of sleepygrass with varying distance from one focal population known for its high toxicity levels near Cloudcroft, NM, USA. For some of these populations, we sampled individual plants twice within the same growing season and over successive years (2001-2004). We also determined infection levels in each population. In general, all populations were highly infected, but infection levels were more variable near the focal population. Only infected plants within populations near the Cloudcroft area produced alkaloids. The ergot alkaloid, ergonovine, comprised the bulk of the alkaloids, with lesser amounts of lysergic and isolysergic acid amides and ergonovinine alkaloids. Levels of all alkaloids were positively correlated among individual plants within and between growing seasons. Infected plants that produced no alkaloids in one year did not produce any alkaloids within the same growing season or in other years. Levels of alkaloids in sleepygrass populations declined steeply with distance from the Cloudcroft population, although infection levels increased. Infected plants in populations in northern New Mexico and southern Colorado produced no alkaloids at all, despite 100% infectivity. Our results suggest that only specific Neotyphodium haplotypes or specific Neotyphodium/grass combinations produce ergot alkaloids in sleepygrass. The Neotyphodium haplotype or host-endophyte combination that produces toxic levels of alkaloids appears restricted to one locality across the range of sleepygrass. Because of the wide variation in alkaloid levels among populations, interactions between the endophyte and host, and consequences for herbivores, competitors, and pathogens and other components of the community are likely to vary widely across the geographic range of this native grass.