Cultures of Cylindrotheca sp. were isolated from the Subantarctic water (SAW) off the coast of New Zealand in 2023 and exposed to a simulated marine heatwave. Thermal performance curve experiments were performed for Cylindrotheca sp. at two time points after exposure to a marine heatwave: T1: immediately after heatwave exposure, and T2: 5 weeks after heatwave exposure. At the end of the thermal performance curve experiments, 1mL of each replicate was preserved using 25% Glutaraldehyde to a final concentration of 0.96% and stored at 4°C. A subsample of preserved cells was mounted on a phytoplankton counting chamber (Phycotech), and cell size was measured using an Evident LC35 camera and LCMicro software connected to an inverted Olympus IX51 microscope. Two measurements were made for Cylindrotheca sp. cells: both the length and the width of the chlorophyll-containing cell centre. The volume of the cell-centres was calculated based on the geometric model of a prolate sphere as described in Hillebrand et al. (1999). The data was collected in 2024 as part of a Master's research project investigating the impacts of marine heatwaves on subantarctic phytoplankton.