Neuroligins are postsynaptic cell-adhesion molecules that are thought to specify synapse properties. Previous studies showed that mutant mice carrying an autism-associated point mutation in neuroligin-3 (NL3) exhibit social interaction deficits, enhanced inhibitory synaptic function, and increased staining of inhibitory synaptic puncta without changes in overall inhibitory synapse numbers. In contrast, mutant mice lacking neuroligin-2 (NL2) displayed decreased inhibitory synaptic function. These studies raised two relevant questions. First, does NL2 deletion impair inhibitory synaptic function by altering the number of inhibitory synapses, or by changing their efficacy? Second, does this effect of NL2 deletion on inhibition produce behavioral changes? We now show that although NL2-deficient mice exhibit an apparent decrease in number of inhibitory synaptic puncta, the number of symmetric synapses as determined by electron microscopy is unaltered, suggesting that NL2 deletion impairs the function of inhibitory synapses without decreasing their numbers. This decrease in inhibitory synaptic function in NL2-deficient mice correlates with a discrete behavioral phenotype that includes a marked increase in anxiety-like behavior, a decrease in pain sensitivity, and a slight decrease in motor coordination. This work confirms that NL2 modulates inhibitory synaptic function and is the first demonstration that global deletion of neuroligin 2 can lead to a selective behavioral phenotype.