We report the discovery of a large anomaly in the isotopic composition of Mg in a Ca-Al rich chondrule from the Allende meteorite. This anomaly is manifest independently of instrumental fractionation and is due to an enrichment of about 1.3 percent in 26Mg while the abundances of 25Mg and 24Mg are terrestrial in value. There is a strong correlation in this chondrule between the 26Mg excess and the Al/Mg ratio so that the most plausible cause of the anomaly is the in situ decay of now extinct 26Al (τ½ = 0.72 × 106 yr). Mineral phases extracted from a Ca-Al-rich aggregate have distinct Al/Mg but show identical, small Mg anomalies which are apparent after correction for fractionation (δ26Mg = 0.3%). These data indicate that this aggregate was isotopically homogenized in a high Al/Mg environment after the decay of 26Al had occurred or that some of the Mg anomalies are due to effects other than in situ decay of 26Al.