Morphometric, electrophoretic, and immunological procedures were used to probe the structural and physiological differences between triazine-resistant (R) and susceptible (S) isolines of canola (Brassica napus L.). The R biotype exhibited increased grana stacking and decreased amounts of starch compared to the S biotype. Likewise, characters associated with an increase in grana stacking (lower chlorophyll a/b ratio, increased chlorophyll a/b light-harvesting complex, and relatively lower amounts of the P700 chlorophyll a protein and chloroplast coupling factor) were all observed in the R isoline of canola. Proteins which occur with approximately equal frequency in grana and stroma lamellae (plastocyanin, cutochrome f) or present only in the stroma (ribulose 1,5-bisphosphate carboxylase/oxygenase) were not quantitatively different in the two biotypes. Gross anatomical parameters (volume of epidermis, palisade mesophyll, spongy mesophyll, and air space) were similar in the two isolines. Thus, the triazine-resistance mutation does not confer a shade-type anatomy despite the chloroplast changes that are characteristic of shade biotypes or shade adaptions. These data indicate that the differences in chloroplast structure noted previously in comparisons of nonisonuclear R and S weed biotypes reflect differences in the triazineresistance factor rather than characters unrelated to triazine resistance.