Pistachio (Pistacia vera L.) progeny from the California cultivar improvement program were evaluated at two locations for their resistance to alternaria late blight [Alternaria alternata (Fries) Keissler] from 1995 to 1997. Large differences between seedlings were observed for disease resistance. Narrow sense heritabilities were calculated from half-sib analysis of 20 open pollinated families. Heritabilities of 0.48 and 0.11 at Kearney Agricultural Center were observed in 1995 and 1997, respectively. Narrow-sense heritability estimates were 0.56 and 0.54 at the Wolfskill Experimental Orchard near Davis for 1995 and 1997, respectively. Differences among progeny families to alternaria late blight infection were highly significant and were associated with the female parents. Greenhouse inoculation experiments were not strongly correlated with field results. Fifty-eight highly resistant seedlings were identified for use in future cultivar improvement efforts.