BL Lacertae was detected by the EGRET instrument on CGRO at the 10.2 \sigma level with an average flux of 171 \pm 42 \times 10 ^ { -8 } { photons cm ^ { -2 } s ^ { -1 } } at energies > 100 MeV during the optical outburst of 1997 July .
This flux is more than 4 times the previously highest level .
Within the July 15-22 observation there was a dramatic factor of 2.5 increase in the gamma-ray flux on July 18.75–19.08 , apparently preceding by several hours a brief optical flare .
The gamma-ray flux decreased to its previous level within 8 hrs and the optical flux decreased to its prior level in less than 2 hrs .
The gamma-ray photon spectral index of 1.68 \pm 0.12 indicates that the spectrum during the 7-day observation was harder than the previous detection .