Amyotrophic Lateral Sclerosis (ALS) is the most common adult onset motor neuron disease. ALS patients experience a rapid loss of motor function over the course of 2-5 years due to motor neuron death and neuromuscular junction (NMJ) deterioration. Approximately 20% of familial ALS cases are caused by mutations in superoxide dismutase-1 (hSOD1), and overexpression of mutant hSOD1 alleles has been used to model ALS in Drosophila, C. elegans, zebrafish, and mice. Overexpression of mutant hSOD1 in mice causes early lethality, progressive loss of motor function, NMJ deterioration, and motor neuron death, but overexpression of WT hSOD1 mimics several of these ALS-like phenotypes as well. The fact that WT hSOD1 overexpression can cause motor neuron death and impair motor function raises the possibility that high levels of SOD1 via overexpression may not accurately recapitulate the pathogenesis of ALS. To model ALS, the Reenan lab used homologous recombination to place synonymous disease causing dSod1 alleles under the control of the endogenous dSod1 promoter. Our characterization demonstrates that two mutations, dSod1G85R and dSod1H71Y, cause 1) early lethality, 2) a loss of neuromuscular junction integrity, 3) electrophysiological defects and 4) a progressive loss of motor function. 1) Both alleles show profound lethality at eclosion with 100% of dSod1G85R and 75% of dSod1H71Y failing to fully eclose. The remaining 25% of dSod1H71Y flies that eclose show a reduced lifespan with a median survival of 14 days and a reduction in climbing ability as they age. 2) To identify why these dSod1 mutants fail to eclose, we examined nerve integrity in legs and the abdominal NMJ that both play an important function in successful eclosion. We found that dSod1G85R and dSod1H71Y mutants have disrupted leg nerve integrity and NMJ structure. 3) Furthermore, electrophysiological recordings from the adult abdominal NMJ show a decrease in mEPSP frequency, consistent with the observed decline in NMJ integrity. 4) To investigate whether these defects were progressive, we studied the motor function of larvae and their NMJs. We found that late 3rd instar larvae, but not mid 3rd instar larvae, have a profound crawling defect. Interestingly, the neuromuscular junctions of late 3rd instar larvae are largely normal in appearance and electrophysiological function, suggesting that other changes in the motor circuit account for the decline in motor function. Overall, we observed defects in 1) survival, 2) NMJ integrity, 3) electrophysiology, and 4) motor function. Given the similarity between the dSod1 knock-in phenotypes and ALS patient symptoms, we anticipate that this model will be a valuable tool for ALS research.