The effect of 100 ppm addition of phosphorus on primary silicon particle number density per unit area N-A and corresponding interparticle spacing lambda is reported for a Bridgman solidified Al-20 wt%Si base alloy. The phosphorus (added as Al-Fe-P base or Al-Cu-P alloys) results in a factor of 3 increase in N-A and a factor of 2 reduction in lambda for the range of conditions studied. In its absence the results conform to lambda (T) over dot(1/3) = 256 +/- 24 mum (K/s)(1/3) where (T) over dot is cooling rate during solidification in good agreement with earlier data. When published data on the effect of 0.02 to 0.2 wt%P are included the combined results are well represented by lambda (T) over dot(1/3) = 250 - 215 (wt%P)(0.17) (lambda in mum, (T) over dot in K/s).