A flow injection analysis (FIA) of phosphorus using coprecipitation with beryllium hydroxide was performed. An on-line preliminary concentration method using a filter tube was developed and this method was applied to the determination of phosphorus in steel. Phosphorus was coprecipitated with beryllium hydroxide by increasing the pH of the medium in the flow tube, and the precipitate was collected with the filter tube (pore size: 1.0 mm, inner diameter: 1.0 mm, length: 7 cm). The precipitate collected was eluted with an acid. The resulting eluate was allowed to react with ammonium molybdate in the presence of antimony, and was then reduced with ascorbic acid. The amount of the phosphorus-antimony-molybdenum ternary complex thus formed was determined spectro-photometrically to be 890 nm. Concentration was performed for 10 min at a flow rate of 0.75 mL/min and the acid used for elution was 1 M HNO 3 . During sample preparation, matrix iron was reduced with sodium hydrogen sulfite and masked with EDTA. The concentration process yielded a sensitivity of analysis about 25 times higher than that of conventional methods. In 0.03 g of steel sample, the method proposed here gave measurements equivalent to those of the phosphorus contents in standard steel samples , and were thus in good agreement with the guaranteed values. The possible range of phosphorus determination is from 0.0021 to 0.017 % in steels.