Triazine-resistant (R) biotypes of several dicot weed biotypes have been shown to differ from triazine-susceptible (S) biotypes both structurally and biochemically. To this point, no R monocot has been examined, however. In this report, we examine the ultrastructure and physiology of three biotypes of Poa annua L.: a S biotype, a R biotype, and a “m ixed” (M) biotype that displays a level of resistance just slightly less than the R biotype, as well as a unique fluorescence in the socalled M region of the fluorescence induction curve. Like the dicots investigated previously, the R and M biotypes have larger grana stacks, and more light-harvesting chlorophyll alb protein than the S biotype. These are similar to the differences described between “shade-type” and “suntype” chloroplasts although anatomical parameters, that normally vary between sun and shade type plants, do not appear to be different in the three biotypes. These data indicate that the structural and biochemical m odifications of the chloroplast reported previously in R biotypes of dicots are also found in R biotypes of m onocots and further indicate that these are natural consequences of the slower electron transport found in R biotypes.