Many stages through an aquaculture facility are affected by water quality. A knowledge of the quality and physical characteristics of the culture water is therefore required if the many unit processes involved are to be optimized. Water quality requirements are well documented, but little work has been conducted regarding the physical characteristics, such as particle size distribution. In recent years the analytical technology in this field has improved greatly. Several methods of characterizing suspended particle size were tested in this study. These included: size fractionation using sieves; laser diffraction; size fractionation using membranes; and characterization using the Coulter registered method. The latter two techniques proved most suitable for aquaculture applications and are described. Fractionation, using membranes, into eight particle size ranges between larger than 200 mu m and less than 0.45 mu m diameter, proved inaccurate in terms of the particle sizes captured, but was a useful method when combined with chemical analyses conducted to determine the location of associated nutrients such as phosphorus and nitrogen compounds. A Coulter Multisizer particle size analyser has been used during three years for a variety of applications, but primarily for the characterization of aquaculture effluents. The operational procedure and representative results are described. The mean random error, associated with the replicate counting of the total number of particles in 28 particle size analyses carried out in duplicate, was 25.1%. This was within the size range 8.4 - 269.2 mu m and total number range 11 - 11 283 particles, in 6 ml samples. A mean replication error of 32.4% was obtained from measurements of the suspended solids concentration in 16 of the previous 28 samples, using standard 0.45 mu m membranes. Suspended solids loadings were within the range 1.4 - 8.4 mg/l. It is likely that the use of particle size characterization techniques will greatly increase in future years. On their own, such techniques are limited in practical applications, but when combined with other forms of analysis, such as nutrient concentration studies, they become more powerful.