Well-defined mu-[poly(ethylethylene)] [poly(ethylene oxide)] [poly(gamma-methyl-epsilon-caprolactone)] (mu-EOC) miktoarm star terpolymers were prepared by a combination of two successive living anionic polymerizations and one controlled ring-opening polymerization (ROP). A mid-hydroxyl-functionalized poly(ethylethylene)-block-poly(ethylene oxide) (PEE-PEO) diblock copolymer served as a macroinitiator for the aluminum-mediated ROP of gamma-methyl-epsilon-caprolactone (MCL), leading to a family of mu-EOC triblock copolymers having monomodal molecular weight distributions and controlled PMCL weight fractions. Micelle structures formed in water from mu-EOC containing various lengths of PMCL were examined by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering. Increasing the fraction of hydrophobic PMCL resulted in a transition from spherical to wormlike to approximately spherical micelles. Although the multicompartment cores of the mu-EOC micelles could not be directly resolved by cryo-TEM, small-angle X-ray scattering analysis of a representative PEE-PMCL diblock copolymer showed clear microphase separation, strongly suggesting the existence of multicompartment cores in the mu-EOC micelles. We speculate that the transition of mu-EOC micelles from worms to spheres can be attributed to the connection of the three immiscible blocks at one junction and documented micellar transitions in analogous PMCL-PEO diblock micelles.