<p>Brominated and iodinated methanes impact atmospheric chemistry, particularly through ozone depletion, but the environmental factors controlling their production by marine phytoplankton are not fully understood. This study examined how different light intensities (30, 60, 90, and 120 µmol photons m^− 2 s^− 1) affect the growth and halomethane production by the marine diatom <i>Achnanthes subconstricta</i>. Cultures were incubated under full-spectrum light, and concentrations of CHBr₃, CHBr₂Cl, CHBrCl₂, CH₂I₂, CH₂ClI, and CH₂BrI were measured using purge-and-trap gas chromatography–mass spectrometry. Phytoplankton growth, assessed by chlorophyll <i>a</i> concentration, increased with light intensity. Among brominated methanes, CHBr₃ and CHBr₂Cl were generally more abundant, and CHBrCl₂ was least abundant. Similarly, CH₂I₂ was generally the dominant iodinated methane, followed by CH₂ClI and CH₂BrI. The production rate ratios of CHBr₃ : CHBr₂Cl : CHBrCl₂ and CH₂I₂ : CH₂ClI : CH₂BrI were 1.8 : 1.7 : 1 and 5.2 : 2.0 : 1, respectively, at 120 µmol photons m^− 2 s^− 1 during the exponential phase. CHBr₃ production rates normalized to chlorophyll <i>a</i> were 2.13, 3.12, 9.49, and 7.24 nmol (g chlorophyll <i>a</i>)⁻¹ d^− 1 at 30, 60, 90, and 120 µmol photons m^− 2 s^− 1, respectively. Similarly, CH₂I₂ production rates normalized to chlorophyll <i>a</i> were 5.47, 2.53, 10.5, and 29.8 nmol (g chlorophyll <i>a</i>)⁻¹ d^− 1 at the same light intensities. These results demonstrate that halomethane production in <i>A. subconstricta</i> is markedly affected by light intensity, with distinct patterns observed for different compounds. The findings suggest that <i>A. subconstricta</i> may play a significant role in marine halocarbon emissions, with production that varies depending on light conditions and growth phase.&#xa0;</p>