Hepadnaviral replication requires the concerted action of the polymerase and core proteins to ensure selective packaging of the RNA pregenome into nucleocapsids. Virus assembly is initiated by cis-preferential binding of polymerase to the encapsidation signal ϵ, present on pregenomic RNA. Using the duck hepatitis B virus (DHBV) model, we analyzed how core protein is recruited to the RNA/polymerase preassembly complex. Two sets of trans-complementation assays were performed in cotransfected hepatoma cells. First, a replication-competent DHBV construct was tested for its ability to rescue replication of genomes bearing mutations within the core region. Self-packaging of wild-type pregenomes was more efficient than cross-packaging of core-deficient pregenomes, and this bias was strongly enhanced if mutant pregenomes coded for self-assembly–competent, but packaging-deficient, core proteins. Second, the site of wild-type core protein translation, i.e., pregenomic RNA (cis) or separate messenger RNA (trans), was analyzed for its effect on the phenotype of a previously described dominant-negative (DN) DHBV core protein mutant. This mutant forms chimeric nucleocapsids with wild-type core proteins and blocks reverse transcription within most, but not all, mixed particles. Strikingly, suppression of viral DNA synthesis by the mutant increased 100-fold when wild-type core protein was provided in trans. Our results suggest that recruitment of core protein to the DHBV preassembly complex occurs in a cis-preferential manner. This mechanism may account for the leakiness of DN DHBV core protein mutants targeting reverse transcription.