The phase behavior and aggregates of a catanionic system have been investigated. The anionic surfactants in the mixed systems were sodium N‐alkylmaleimidepimaric carboxylate (Cn‐MPA‐Na, n=12, 14, 16), which were prepared from rosin and contain a large rigid skeleton and a flexible alkane chain, and the cationic surfactant was cetyltrimethylammonium bromide (CTAB). The phase behavior of the C14‐MPA‐Na/CTAB system transformed sequentially from a viscoelastic solution to an aqueous surfactant two‐phase system (ASTP), an aqueous surfactant three‐phase system (AS3P), and an anisotropic homogeneous phase as the concentration of C14‐MPA‐Na was continuously increased from 10 to 35 mm. The C16‐MPA‐Na/CTAB system showed similar phase behavior, whereas the C12‐MPA‐Na/CTAB system did not form the AS3P system. The corresponding microstructures in the different phases were investigated by using rheology and cryogenic transmission electron microscopy (Cryo‐TEM). The aggregates in the viscoelastic solutions are thread‐like, annular, and worm‐like micelles. The microstructures in the upper phase of the ASTP are worm‐like micelles, and in the lower phase are spherical and rod‐like micelles. The aggregates in the upper and lower phases of the AS3P are worm‐like micelles and spherical and rod‐like micelles, respectively. The aggregates in the middle phase of the AS3P and the anisotropic homogeneous phase are sponge‐like micelles. The clear Cryo‐TEM images of the sponge‐like micelles are presented.