The ribulose 1,5-diphosphate carboxylase (RUDP Case E.C. 184.108.40.206) activity of late log phase Anabaena cylindrica Lemm. was measured in vitro in fractions obtained by sucrose density gradient centrifugation. Two peaks of enzymic activity were obtained. One, accounting for about 80% of the total measurable activity occurred at the top of the gradient and appeared to be soluble activity; the second showed maximum activity in the 55% (w/w) sucrose fraction and represented 20% of the total activity. When the distribution of RUDP Case was assayed by immunoprecipitation using antiserum to RUDP Case from Euglena gracilis, the corresponding values were 59% and 41%. Electron microscopy of the various fractions showed that polyhedral bodies, which are sites of RUDP Case activity in other autotrophic prokaryotes, were also most abundant in the 55% (w/w) sucrose fraction.