Millipore (R) membrane filters (0.45 μm), Gelman glass fiber filters (Type A-E), and filter papers (Whatman No. 42 and No. 1) were all suitable for preparing pond water samples for orthophosphate determination. The stannous chloride method was just as accurate and precise, but more sensitive, than the ascorbic acid method for orthophosphate. However, the longer stability of the colored sol made the ascorbic acid method more suitable for analyzing pond waters for filtrable orthophosphate. Concentrations of filtrable orthophosphate changed rapidly during storage of unpreserved samples in polyethylene bottles. When a high degree of accuracy is necessary, samples should not be stored for more than 1 hour before analysis.