  Gamma-rays from cosmological sources contain information about gamma-ray
interactions. Standard model and non-standard model photon interactions along
the path between the source and the observer can lead to changes in the energy
or state of the photons, which in turn alters the observed energy spectrum of
the source. In general, these interactions are a function of photon energy as
well as source distance. Here we show how existing high energy (50 GeV --
10TeV) gamma-ray observations of blazars can be used to constrain the coupling
of axion-like-particles (ALPs) to the photon. The same ALP-photon coupling that
has been invoked to explain the observations of TeV blazars beyond their pair
production horizon is shown to have an effect of the data set of \textit{Fermi}
blazars.
