The principle of the method specified is isokinetically withdrawing a known volume from a moving gas stream, passing the sampled gas through a filter medium to remove particulate matter (including fibres), treating the filter to make it transparent when viewed under a microscope and counting the number of fibres in a precise number of fields viewed using a phase-contrast optical microscope. The method may be used to determine fibre concentrations in flowing gas streams in ducts, chimneys, or flues from a wide range of industrial processes.