Semiconductor, quantum dot (QD) nanoparticles (including CdSe/ZnS, CdTe/ZnS, and CdSe) were encapsulated within crosslinked shells of amphiphilic polystyrene-block-poly(acrylic acid) block copolymer. Transmission electron microscopy revealed that each particle was surrounded by a uniform layer of copolymer, and that the average diameter of the resulting QD-core micelles was between 25 and 50 nm, depending on the conditions of particle assembly. Overall, we found that aqueous suspensions of these QDs were substantially more stable to heat and pH than particles with other surface preparations; we argue that the enhanced stability is due to the uniform, hydrophobic coating of polystyrene around each particle and the reinforcement of this layer by shell-crosslinking. The biocompatibility of these particles was investigated by microinjection of particle suspension into live zebrafish embryos. The particles permanently stained the fish vasculature, but did not interfere with the normal development of the fish. We propose that QDs encapsulated in crosslinked block-copolymer shells allow QDs to be used in biological or biotechnological protocols requiring harsh reaction conditions.