  We report on the search for spectral irregularities induced by oscillations
between photons and axionlike-particles (ALPs) in the $\gamma$-ray spectrum of
NGC 1275, the central galaxy of the Perseus cluster. Using six years of Fermi
Large Area Telescope data, we find no evidence for ALPs and exclude couplings
above $5\times10^{-12}\,\mathrm{GeV}^{-1}$ for ALP masses $0.5 \lesssim m_a
\lesssim 5$ neV at 95% confidence. The limits are competitive with the
sensitivity of planned laboratory experiments, and, together with other bounds,
strongly constrain the possibility that ALPs can reduce the $\gamma$-ray
opacity of the Universe.
