Abstract Bromoform (CHBr 3 ) has an important role in transporting bromine from the ocean to the atmosphere, and released bromine catalyses ozone depletion. In temperate ocean waters, a number of studies have observed or presumed CHBr 3 production. Here, we studied the ability of marine phytoplankton to produce CHBr 3 in cultures of temperate phytoplankton. A temperate marine diatom and a cyanobacterium, Ditylum brightwellii CCMP 358 and Synechococcus sp. CCMP 1334, respectively, were incubated at 24 °C and the concentrations of brominated methanes in the cultured samples were determined using purge and trap gas chromatograph–mass spectrometry. The axenic cultures of the diatom exhibited a remarkable rate of CHBr 3 production, ~ 200 nmol (g chlorophyll a ) − 1 h − 1 , which was several times higher than that for cold water diatoms reported previously. The cyanobacterium also produced CHBr 3 , with a production rate of ~ 1 nmol (g chlorophyll a ) − 1 h − 1 , shows that two diverse phytoplankton can produce CHBr 3. Both CHBr 2 Cl and CHBrCl 2 were also produced in the D. brightwellii culture, but only CHBr 2 Cl was produced in the culture of the Synechococcus sp. An incubation experiment with 13 CHBr 3 revealed that there was no 13 CHBr 3 degradation (or the formation of 13 CHBr 2 Cl and 13 CHBrCl 2 ) in the cultures of these two phytoplankton, and these results suggested that both diatoms and cyanobacteria could produce chlorinated methanes. Our results suggest that brominated methanes such as CHBr 3 are produced by temperate phytoplankton and that phytoplankton is a significant source of CHBr 3 in the temperate open ocean.