A new virus that infects the harmful algal bloom-forming microalga Phaeocystis globosa was isolated from surface water in the English Channel off the coast of Plymouth, UK, in May 2001. Phylogenetic analysis of the DNA polymerase gene revealed the virus isolate, designated PgV-102P, belongs to the family Phycodnaviridae, a group of large double-stranded DNA viruses known to infect algae. Basic characterization of PgV-102P revealed it was a lytic virus with a relatively slow culture lysis period of 10-days. The genome size (176 kbp) and capsid diameter (98 nm) of PgV-102P fall at the bottom end of the range expected for phycodnaviruses. Interestingly, PgV-102P did not cluster with other P. globosa viruses; instead, it was more closely related to other prymnesioviruses that infect the marine prymnesiophyte Chrysochromulina brevifilum. We discuss the effectiveness of DNA polymerase as a diagnostic marker. Although it is ideal for determining what family or even genus an algal virus belongs to, it is clear that the DNA polymerase gene does not have sufficient resolution when looking for relationships within algal virus genera.