  We report the AGILE detection and the results of the multifrequency follow-up
observations of a bright $\gamma$-ray flare of the blazar 3C 279 in June 2015.
We use AGILE-GRID and Fermi-LAT $\gamma$-ray data, together with Swift-XRT,
Swift-UVOT, and ground-based GASP-WEBT optical observations, including
polarization information, to study the source variability and the overall
spectral energy distribution during the $\gamma$-ray flare. The $\gamma$-ray
flaring data, compared with as yet unpublished simultaneous optical data which
allow to set constraints on the big blue bump disk luminosity, show very high
Compton dominance values of $\sim 100$, with a ratio of $\gamma$-ray to optical
emission rising by a factor of three in a few hours. The multi-wavelength
behavior of the source during the flare challenges one-zone leptonic
theoretical models. The new observations during the June 2015 flare are also
compared with already published data and non-simultaneous historical 3C 279
archival data.
