We used the Matlock Periphytometer (Matlock et al., 1998) to measure in situ nutrient limitations and trophic status at five stream sites in the Bosque River Watershed in north-central Texas during July 1997. Periphytic chlorophyll a production from the Matlock Periphytometer was also used as an indicator of baseline primary productivity and of maximum primary productivity (MPP) in response to nutrient enrichment (nitrogen and phosphorus). The measured MPP was interpreted to represent the rate of periphytic growth when nutrients are not limiting. We developed a lotic ecosystem trophic status index (LETSI) using the ratio of baseline primary productivity to MPP. Nitrogen (N) and phosphorus (P) limitations were evaluated using a modified LETSI as the ratio of either N or P enriched growth to MPP. The LETSI is by definition a functional index, and may provide a classification tool for lotic ecosystem trophic status. Using the LETSI indices, we observed differences in nutrient limitations in the streams and detected co-limitations of nitrogen and phosphorus at two of the stream sites.