Prasinoviruses are among the largest viruses (>200 kb) and encode several hundreds of protein coding genes, including most genes of the DNA replication machinery and several genes involved in transcription and translation, as well as transfer RNAs (tRNAs). They can infect and lyse small eukaryotic planktonic marine green algae, thereby affecting global algal population dynamics. Here, we investigate the causes of codon usage bias (CUB) in one prasinovirus, OtV5, and its host Ostreococcus tauri, during a viral infection using microarray expression data. We show that 1) CUB in the host and in the viral genes increases with expression levels and 2) optimal codons use those tRNAs encoded by the most abundant host tRNA genes, supporting the notion of translational optimization by natural selection. We find evidence that viral tRNA genes complement the host tRNA pool for those viral amino acids whose host tRNAs are in short supply. We further discuss the coevolution of CUB in hosts and prasinoviruses by comparing optimal codons in three evolutionary diverged host-virus-specific pairs whose complete genome sequences are known. © The Author(s) 2013. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.