Negative growth (shrinkage) at moulting in male and female West Coast rock lobster Jasus Ialandii under field conditions is documented. A decline in growth rates of male rock lobsters in the late 1980s/early 1990s is reflected in the steep rise in the frequency of zero and negative growth increments at moulting in four areas along the South African west coast. Shrinkage is considered to be a direct result of adverse environmental conditions and not a physiological mechanism evolved to overcome defined periods of hardship. Mean growth rates of male lobsters incorporating negative growth increments are presented and the implications for management discussed. Slow growth resulted in a decrease in female size at 50% maturity in all four study areas, suggesting that juvenile as well as adult growth rates have been adversely affected.