Quick-freeze deep-etch of acidosomes show the results of carrier vesicle fusion with the acidosomes. Both the E-face and P-face of the acidosomes appear to have two areas, the larger area on the E-face has few IMPs but has pits left by the removal of IMPs during fracturing and another area that is smooth with fewer pits that resembles a fused carrier vesicle membrane. A line marking the fusion site between these two vesicles is evident. The P-face of the acidosome has many IMPs while the fused carrier vesicle membrane is smooth. Presumably these two membrane areas will mix with time. Thus acidosomes, which are somewhat similar in appearance to multivesicular bodies in higher organisms, are late endosomes and deliver their membrane, their contents and their V-ATPase complexes to the phagosome to transform the phagosome to the phagoacidosome. TEM taken on 7/29/88 by C. Schroeder with Zeiss 10A operating at 80kV. Neg. 24,800X. Published in J. Cell Sci. 106:411-422, 1993. Adapted with permission.