Female guppies were fed diets containing different levels and ratios of calcium (0.03%, 0.59% and 1.28%) and phosphorus (0.05%, 0.53% and 1.23%) for 12 weeks. Growth and feed conversion rates were found to be positively correlated with dietary levels of P but not of Ca. Lower levels of vertebral ash, Ca and P were also seen in fish given 0.05% P diets. The dietary phosphorus level required for optimum growth and mineralisation was found to fall between 0.53% and 1.23%.