Northern quahogs Mercenaria mercenaria were grown in trays suspended in the York River, Virginia from November 1954 (4 months old) to December 1970 (16 years old). Measurements of shell length (mm) were made at least once a year from 1954 to 1970 and measurements of shell width (mm) were made in November 1962 and August 1965 and then once per year from 1967 through 1970. Quahog densities within the trays ranged from 1500 m⁻² (November 1954 to October 1955) to 269 m⁻² (November 1955 to December 1970). Quahog shell length (mm) increased with age (yr) and was described with a Von Bertalanffy growth model (coefficient of determination = 0.93). Most of the growth in shell length occurred in the first 6 years with clams reaching shell lengths of 58 mm by November 1960 and maximum shell lengths of 77–84 mm by 1963 (age 9). Shell length:shell width relationships were similar for tray held and wild quahogs collected from the York River during 1967 to 1970. The observed annual shell length growth increment decreased exponentially with quahog age. Standardized shell length growth index (SGI) values for 11 of the 15 years for which data are available describe better than expected quahog growth trends although there was no clear relationship between SGI and average annual surface temperature or average growth period (water temperature >7°C, typically March to November) surface temperature in the York River between 1955 and 1967.