The time course of total citrate synthase activity in castor bean (Ricinus communisL., type Sanzibariensis) endosperm showed a 7‐fold increase during the initial 5 days of germination and a decrease thereafter. All citrate synthase activity in the ungerminated seeds was due to the mitochondrial isoenzyme. After two days of germination the glyoxysomal isoenzyme began to appear. After 5 days the glyoxysomal citrate synthase represented 50 to 55% of the total activity and the mitochondrial enzyme the remainder. This was estimated from (a) inactivation of the glyoxysomal citrate synthase by 5,5′‐dithiobis(2‐nitrobenzoic acid); (b) solid phase adsorption of the glyoxysomal synthase by a specific antiserum; (c) separation of isoenzymes by (NH4)2SO4gradient solubilization.The increase of both citrate synthases during the initial 4 days of germination could be prevented by 10 μg cycloheximide ml−1, but not by 40 or 400 μg chloramphenicol ml−1, indicating a synthesis on 80 S ribosomes. Actinomycin D completely inhibited the appearance of the glyoxysomal enzyme while the mitochondrial enzyme was not affected. Antisera against the two isoenzymes revealed major structural differences between two citrate synthases, however, also some common determinants. No cross‐reaction was observed with the citrate synthase from pig