Significance Decades of research from many laboratories has established a model in which phosphorylation of the GluA1 AMPA-type glutamate receptor subunit plays a significant role modulating long-term potentiation and depression, homeostatic and neuromodulator-regulated plasticity, spatial memory, fear/extinction, and appetitive incentive learning. However, a recent study suggests that GluA1 phosphorylation is exceedingly low, even in synaptic fractions. Here, we address this controversy using in vitro and in vivo techniques. We find a large fraction of excitatory synapses are positive for phosphorylated GluA1. Moreover, phosphorylated species make up a significant fraction of the population and are highly responsive to numerous physiologically relevant stimuli. This characterization reaffirms a large body of work defining a prominent role of AMPA receptor phosphorylation in synapse biology and synaptic plasticity.