Diatoms are one of the most significant primary producers in the ocean, and the importance of viruses as a potential source of mortality for diatoms has recently been recognized. Thus far, eight different diatom viruses infecting the genera Rhizosolenia and Chaetoceros have been isolated and characterized to different extents. We report the isolation of a novel diatom virus (ClorDNAV), which causes the lysis of the bloom-forming species Chaetoceros lorenzianus, and show its physiological, morphological, and genomic characteristics. The free virion was estimated to be ∼34 nm in diameter. The arrangement of virus particles appearing in cross-section was basically a random aggregation in the nucleus. Occasionally, distinctive formations such as a ring-like array composed of 9 or 10 spherical virions or a centipede-like array composed of rod-shaped particles were also observed. The latent period and the burst size were estimated to be <48 h and 2.2 × 10(4) infectious units per host cell, respectively. ClorDNAV harbors a covalently closed circular single-stranded DNA (ssDNA) genome (5,813 nucleotides [nt]) that includes a partially double-stranded DNA region (979 nt). At least three major open reading frames were identified; one showed a high similarity to putative replicase-related proteins of the other ssDNA diatom viruses, Chaetoceros salsugineum DNA virus (previously reported as CsNIV) and Chaetoceros tenuissimus DNA virus. ClorDNAV is the third member of the closed circular ssDNA diatom virus group, the genus Bacilladnavirus.