Rock lobsters can be exposed to poor water quality during all stages of handling and holding prior to going to market. Poor water quality reduces the time a lobster can be held alive and how many animals can be held in a system and thus may reduce profit. The quality of water can be assessed using many different measurements, with two of the most important being oxygen and ammonia (a form of nitrogen). An earlier FRDC funded study investigated oxygen and how it influenced the holding of rock lobsters. However, prior to the current study there was very limited understanding of the harmful effects of ammonia to rock lobsters. Ammonia can accumulate in holding and transport facilities via natural release of ammonia from lobsters, and from the bacterial decomposition of faeces, excess feed, and dead animals. Ammonia can be harmful to crustaceans in small amounts (or low concentrations) and even fatal if concentrations get too high. The toxicity of ammonia to aquatic animals becomes greater when other factors such as low dissolved oxygen, low salinity, and/or low pH (acidity of the water) also interact. In liquids, total ammonia comprises un-ionised ammonia (NH3), which is the more toxic component, and ionised ammonia (NH4 +; ammonium) in equilibrium. Lobsters can become stressed (having a higher demand upon their biological systems) during holding and handling but it is uncertain what effect this stress has on the ability of lobsters to tolerate ammonia. This project provided a better understanding of the effect of ammonia and other water quality measurements, on the health of stressed and unstressed lobsters.