Conflict-Free Replicated Data Types (CRDTs) are distributed data types that support optimistic replication: replicas can be updated locally, and updates propagate asynchronously among replicas, so consistency is eventually obtained. This ability to tolerate asynchronous communication makes them ideal candidates to serve as software building blocks in opportunistic networks (OppNets), that is, mobile networks in which the dissemination of information can only depend on unpredicted transient radio contacts between pairs of nodes. In this paper we investigate the problem of implementing CRDTs in an OppNet, and we propose a delta-state-based algorithm to solve this problem. Experimental results confirm that this algorithm ensures the synchronization of CRDT replicas in an OppNet, and that it outperforms a pure state-based synchronization algorithm when dealing with container CRDTs.