We have found an optical/X-ray counterpart candidate for the bright , but presently unidentified , Fermi source 2FGL J1311.7 - 3429 .
This counterpart undergoes large amplitude quasi-sinusoidal optical modulation with a 1.56h ( 5626s ) period .
The modulated flux is blue at peak , with T _ { eff } \approx 14,000 K , and redder at minimum .
Superimposed on this variation are dramatic optical flares .
Archival X-ray data suggest modest binary modulation , but no eclipse .
With the \gamma -ray properties , this appears to be another black-widow-type millisecond pulsar .
If confirmation pulses can be found in the GeV data , this binary will have the shortest orbital period of any known spin-powered pulsar .
The flares may be magnetic events on the rapidly rotating companion or shocks in the companion-stripping wind .
While this may be a radio-quiet millisecond pulsar , we show that such objects are a small subset of the \gamma -ray pulsar population .