To determine if different algal viruses (Phycodnaviridae) share common patterns of seasonal abundance, quantitative PCR methods were developed and applied to monitor the abundances of three different viruses in Lake Ontario, Canada over 13 months. Throughout the year, the abundances of two different phycodnavirus polB gene fragments (LO1b-49 and LO1a-68) varied by more than two orders of magnitude, peaked during the autumn months, and were lowest during the summer. The seasonal abundance patterns of these two virus genes were similar and both were detected in almost every sample, but LO1b-49 was consistently an order of magnitude more abundant than LO1a-68. LO1b-49 reached a maximum abundance of 5413 +/- 312 genes ml(-1), whereas LO1a-68's abundance peaked at only 881 +/- 113 genes ml(-1). Another phycodnavirus polB fragment that was monitored (LO1b-16) was detected in only a few samples, but reached a higher maximum concentration (6771 +/- 879 genes ml(-1)) than either LO1b-49 or LO1a-68. The results of this year-long investigation of virus gene abundances suggests that Lake Ontario's phycodnavirus community is composed of persistent viruses detectable throughout the year and transient viruses present in only a few sporadic samples. The results also suggest that some persistent algal viruses are able to survive at relatively low abundances through several seasons.