Two different crystal forms of the B800–850-antenna complex from Rhodopseudomonas acidophila strain 10050 have been grown. This complex is an integral membrane protein and is isolated as an oligomeric assembly with a molecular weight of approximately 84 kDa. This assembly contains six α/β apoprotein pairs, 18 molecules of bacteriochlorophyll a and nine molecules of carotenoid. The first crystal form has dimensions unit cell a= b= 75· 8 A ̊, c= 97· 5 A ̊ with the space group P4 and diffracts to a resolution of 12· 0 Å. The second crystal form is rhombohedral with dimensions unit cell a= 121· 1 A ̊, α= 60°, space group R32 and diffracts to a resolution of 3· 5 Å. Native data have been processes in both cases, to an R merge value of 9· 0 to 11· 0%. The X-ray data suggest that the asymmetric unit, in both crystal forms, contains one 84 kDa antenna complex.