The in vitro reconstitution of complex biological behavior from simple components is challenging in wet ALIFE. Here, we present the reconstitution of host-parasite oscillation dynamics in vitro using artificial RNA replicators. We used a translation-coupled RNA replication system encapsulated in water-in-oil droplets. This system used an artificial genomic RNA that replicates using the own encoded replication enzyme. During replication, a parasitic RNA, which lost the replicase gene, spontaneously appeared and co-replicated with the genomic “host” RNA, representing an oscillation in population dynamics. The presence of the parasitic RNA induced the evolution of the host RNA and produced diversity in the host RNA. These results demonstrate the important role of a parasitic entity on the evolution of life.