THE juvenal plumage of the Little Blue Heron (Florida caerulea) has always been described as white with dull brownish gray tips to the primaries (Palmer, 1962: 428, and earlier authors). In September 1962, when Dickerman was collecting blood specimens from nestling herons in a large mixed colony at San Blas, Nayarit, Mexico, an assistant brought to the boat for bleeding a large ambulatory nestling, 20-25 days of age, which because of the chestnut suffusion on the crown and back was considered at the time a between the Little Blue Heron and Louisiana Heron (Hydranassa tricolor). In October 1963 both authors visited the same heronry and found several juveniles showing a gradient of characters approaching the earlier specimen. Additional collecting at San Blas in 1964 and 1965 showed that this type of variation was relatively common and regular in occurrence. A total of 13 juveniles was collected. The San Blas specimens vary from those fitting the classical description of the juvenile Little Blue Heron to in.dividuals with the top of the head predominantly chestnut mixed with gray, and with a dingy chestnut cast to the interscapular area and lesser wing coverts. The primaries in such individuals are more extensively tipped with dusky than those of typical juveniles, the alula is marked with black, and the inner secondaries and scapulars are flecked with gray. Intermediate individuals have lesser amounts of chestnut or gray in these areas. The theory was initially suggested by the prominence of chestnut in the extreme individuals, as juvenile Louisiana Herons are extensively chestnut (see the excellent colored illustration by Eckelberry (in Pough, 1951: pl. 15). A total of 15 adult Little Blue and 14 adult and juvenile Louisiana herons collected in the San Blas colony shows no sign of hybrid or intermediate characters. The plumage variation shown by the San Blas nestlings stimulated us to examine all Little Blue Heron specimens in a number of major museums (see Acknowledgments). We found a few juveniles with grayish crowns and interscapulars that approached some of the San Blas birds ,and occasionally a specimen with black markings on the alula (characters not mentioned in literature descriptions of the juvenal plumage), but no juveniles with the chestnut coloration of the extremes among the San Blas population. As our sample of adult and juvenile Louisiana Herons showed no evidence of introgression, we can only conclude that the chestnutcrowned juvenal plumage represents a localized color phase, thus far known only from San Blas.