The synthesis of the C1-type isocitrate lyase found during growth of Pseudomonas MA on methylamine was investigated. It was shown that this enzyme is subject to catabolite repression by preferred carbon sources, e.g., succinate, and by ammonia. The carbon repression can be overcome by cyclic AMP, which was shown to be acting at the transcriptional level. Repression by ammonia is overcome during growth with methylamine as sole nitrogen, but not carbon, source. Uptake experiments showed that the uptake of methylamine from the medium was prevented by ammonia in the presence, but not in the absence, of an alternative carbon source. Measurement of cyclic AMP levels in cells grown on methylamine and on succinate, glycerol, glucose and acetate as carbon sources (with ammonium chloride as nitrogen source) revealed that methylamine-grown cells have the lowest cyclic AMP level despite having the highest C1-type isocitrate lyase activity. Cells grown on acetate with methylamine as sole nitrogen source possess both C1-type and C2-type isocitrate lyase. The results indicate that the synthesis of C1-isocitrate lyase is under control by repression-derepression involving a specific inducer, cyclic AMP, and an effector whose action is related to the nitrogen supply of the cell.