The experimental design and results of an interlaboratory study for dioxin in water is described herein. The interlaboratory study of EPA Method 613 consisted of the replicate analyses of a performance evaluation sample used primarily for determining laboratory competence and subsequent analyses of six sample concentrations and a blank in six different waters (42 samples). Statistical analyses and conclusions in this report are based on analytical data obtained by eleven collaborating laboratories. Participating laboratories were selected based upon technical evaluation of proposals and upon the analytical results of the prestudy samples. Data obtained from the interlaboratory study were analyzed employing US EPA's series of computer programs known as the Interlaboratory Method Validation Study (IMVS) system, which implements the concepts recommended in ASTM Standard D 2777. The statistical analyses included tests for the rejection of outliers, estimation of mean recovery (accuracy), estimation of single-analyst and overall precisions, and tests for the effect of water type on accuracy and precision.