A mutant (NS 458) of Nicotiana sylvestris (Spegazzini and Comes) unable to synthesize leaf starch was isolated in the M(2) generation following ethyl methanesulfonate mutagenesis by testing with iodine. Segregation ratios in reciprocal F(2) progenies showed that the starchless phenotype resulted from a recessive mutation in a single nuclear gene. DEAE-agarose chromatography showed that the mutant is grossly deficient in plastid phosphoglucomutase (EC 2.1.5.1) activity. The structure of the enzyme is changed, as evidenced by increased Michaelis constants and by the prolonged activation period (>40 minutes) observed when the enzyme is assayed in triethanolamine buffer rather than imidazole buffer. The activity of the wild-type enzyme with saturating glucose 6-P alone was 7% of the activity when saturating glucose 1,6-P(2) was also present. The results suggest that glucose 1,6-P(2) is both an effector and a dissociable reaction intermediate. The growth rate of mutant and wild-type plants were not significantly different in continuous light and on an 8-hour dark, 16-hour light cycle and the mutants grew normally under greenhouse conditions. The mutant supports growth during diurnal periods of darkness by vacuolar storage of sugars instead of chloroplast storage of starch. The simplification in metabolism achieved by blocking the diversion of plastid fructose-6-P to starch facilitates the induction of oscillations in CO(2) fixation.