WHEN IS BL LAC NOT BL LAC

We have found broad H alpha and H beta line emission in spectra of BL Lacertae taken on 1995 May 21 and June 01: on those occasions BL Lac d id not meet the defining criteria for the class named after it. The H alpha line luminosity, similar to 2 x 10(41) h(-2) erg s(-1) for H-0 = 100 h km s(-1) Mpc(-1), is significantly above the detection threshol d in older published spectra which do not show the line. Several possi ble explanations for the increase in broad line luminosity are discuss ed. The continuum was at the faint end of its commonly observed range: V similar to 16.0 on 1995 May 21. The polarization, measured on 1995 June 1, was also near the low end of the typical range in BL Lac: simi lar to 5.0% in PA similar to 22 degrees near 7500 Angstrom, rising to similar to 6.1% in PA similar to 15 degrees near 4900 Angstrom. Absorp tion features in the spectrum show the presence of the associated gala xy, but the light is still dominated by an underlying smooth continuum , the intrinsic shape of which is well approximated between 8000 and 4 000 Angstrom by F-nu proportional to nu(-1.7), indicating a synchrotro norigin. We confirm the narrow emission line redshift, z = 0.0686 +/- 0.0004.