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Cyanobacterial distributions are shaped by abiotic factors including temperature, light and nutrient availability as well as biotic factors such as grazing and viral infection. In this study, we investigated the abundances of T4-like and T7-like cyanophages and the extent of picocyanobacterial infection in the cold, high-nutrient-low-chlorophyll, sub-Antarctic waters of the southwest Pacific Ocean during austral spring. Synechococcus was the dominant picocyanobacterium, ranging from 4.7 × 10 3 to 1.2 × 10 5 cells∙mL -1 , while Prochlorococcus abundances were relatively low overall, ranging from 1.0 × 10 3 to 3.9 × 10 4 cells∙mL -1 . Using taxon-specific, single-virus and single-cell polony methods, we found that cyanophages were on average 15-fold, and up to 50-fold, more abundant than cyanobacteria in these waters. T4-like cyanophages (ranging from 1.7 × 10 5 to 6.5 × 10 5 phage·mL -1 ) were 2.7-fold more abundant than T7-like cyanophages (ranging from 3.1 × 10 4 to 2.8 × 10 5 phage·mL -1 ). Picocyanobacteria were primarily infected by T4-like cyanophages with more Synechococcus (4.8%-12.1%) infected than Prochlorococcus (2.5%-6.2%), whereas T7-like cyanophages infected less than 1% of both genera. These infection levels translated to daily mortality in the range of 5.7%-26.2% and 2.9%-14.3% of the standing stock of Synechococcus and Prochlorococcus, respectively. Our findings suggest that T4-like cyanophages are significant agents of cyanobacterial mortality in the cold, low-iron, sub-Antarctic waters of the South Pacific Ocean., (© 2025 The Author(s). Environmental Microbiology published by John Wiley & Sons Ltd.)