A flow injection–inductively coupled plasma–mass spectrometric (FI–ICP–MS) procedure, utilising ultrasonic nebulisation with membrane desolvation (USN/MD), has been developed for the determination of plutonium (Pu) in seawater at fgl⁻¹ concentration levels. Seawater samples (1l), after filtration, were subjected to co-precipitation with NdF₃, followed by ion exchange to enrich Pu and to reject seawater matrix ions and co-existing uranium. The seawater concentrate (1.0ml) was then analysed by FI–ICP–MS. The limit of detection for ²³⁹Pu in seawater based on an enrichment factor of 1000 was 5fgl⁻¹, and precision at the 0.80pgl⁻¹ level was 12% RSD. Accuracy was verified via recovery experiments, and by comparing survey data for the Irish Sea with that derived by standard methodology based on co-precipitation and α-spectrometry. Concentrations for dissolved ²³⁹Pu and ²⁴⁰Pu in the Irish Sea were in the range of 0.267–0.941pgl⁻¹ (0.614–2.164mBql⁻¹) and 0.051–0.196pgl⁻¹ (0.428–1.646mBql⁻¹), respectively.