Three novel classes of the C4 plant Amaranthus edulis are described. (1) A mutant that contains less than 10% of the normal activity and protein of phosphoenol-pyruvate carboxylase. (2) A mutant that is deficient in NAD-malic enzyme activity, but contains the normal two subunits of the enzyme protein. (3) Four mutant plants that accumulate high concentrations of glycine following exposure to air. A common characteristic of all the mutant plants is that they grow poorly in air and exhibit very low rates of photosynthetic CO2 assimilation. The characteristics of the mutant lines are discussed with reference to our current knowledge of the regulation of the C4 photosynthesis pathway. The possibility that C4 plants undergo photorespiratory nitrogen and carbon metabolism when grown in air is considered in detail.