Visual experience scales down excitatory synapses in the superficial layers of visual cortex in a process that provides an in vivo paradigm of homeostatic synaptic scaling. Experience-induced increases in neural activity rapidly upregulates mRNAs of immediate early genes involved in synaptic plasticity, one of which is Arc (activity-regulated cytoskeleton protein or Arg3.1). Cell biological studies indicate that Arc/Arg3.1 protein functions to recruit endocytic machinery for AMPA receptor internalization, and this action, together with its activity-dependent expression, rationalizes a role for Arc/Arg3.1 in homeostatic synaptic scaling. Here, we investigated the role of Arc/Arg3.1 in homeostatic scaling in vivo by examining experience-dependent development of layer 2/3 neurons in the visual cortex of Arc/Arg3.1 knock-out (KO) mice. Arc/Arg3.1 KOs show minimal changes in basal and developmental regulation of excitatory synaptic strengths but display a profound deficit in homeostatic regulation of excitatory synapses by visual experience. As additional evidence of specificity, we found that the visual experience-induced regulation of inhibitory synapses is normal, although the basal inhibitory synaptic strength is increased in the Arc/Arg3.1 KOs. Our results demonstrate that Arc/Arg3.1 plays a selective role in regulating visual experience-dependent homeostatic plasticity of excitatory synaptic transmission in vivo.