wit^A12/wit^C175 transheterozygous mutants are completely lethal. Approximately 50% of animals survive to the pharate stage. The remaining animals die during larval and early pupal stages. Those animals that do survive to pharate adult stage exhibit the head expansion and abdominal peristaltic movements characteristic of normal pre-eclosion behaviour, but do not rupture nor escape from the pupal case. Many pharate adults show a bend in the femur or tibia, the phenotype being more pronounced in the metathoracic leg. A small fraction of the animals also exhibit truncations of the tarsal segments. The general organisation of the nervous system in these mutants is essentially normal. wit^A12/wit^C175 mutant larvae exhibit dramatically reduced transmitter release from presynaptic terminals when the excitatory junctional current (EJC) of body wall muscles examined. The amplitude of EJCs at 0.8 mM Ca²⁺ is significantly reduced in mutants. Most nerve stimuli fail to evoke transmitter release at low extracellular ^Ca<up>2+</up> in mutants. Cooperativity is unaffected in mutants. The amplitude of miniature excitatory junction potentials (mEJPs) is not significantly altered in mutants.