|Abstract||The quagga mussel, cousin of the well-known zebra mussel, is an invasive Eurasian bivalve that has invaded the lake of the United States. Quagga mussels have been extremely successful, and one reason for that is their strong ability to reproduce. Here, an attempt to analyze the quagga mussels' ability to resist polyspermy (multiple sperm fertilizing one egg) when exposed to very high concentrations of sperm was undertaken. Individual animals were separated 24-48 hours before spawning in order to ensure no previous mating had occurred and to help stimulate spawning. Spawning was then fully induced using serotonin and isolated samples were collected after 20 min. The eggs were then fertilized with four different concentrations of sperm (10^6, 10^5, 10^4, and 10^3 cells/mL). At time points 3' and 20', samples were fixed to identify sperm bound and sperm entered, respectively. It was found that 10^6 cells/mL, the eggs lacked the ability to consistently resist polyspermic bindings, but by far, it also had the highest percentage of monospermic incorporations.