Small (∼15 mm) and large (∼30 mm) calcein-marked bay scallops, Argopecten irradians, held for 2, 4, and 6 wk in the laboratory under natural illumination and conditions of high and low flow rates deposited significantly more striae on the surface of the left (dark) shell valve compared with the right (light) shell valve. Small scallops deposited an average of 0.55 stria per day, 0.42 stria per day, and 0.34 stria per day, respectively, during the 2-, 4-, and 6-wk experiments, whereas large scallops had a lower frequency of stria formation (0.20 stria per day, 0.18 stria per day, and 0.17 stria per day, respectively). Striae deposition and interstria distance were highly variable among small A. irradians. No relationship in interstria distance was obvious in A. irradians that deposited the same number of striae during 6 wk (0.45 striae per day) and held under conditions of high flow rate, indicating that stria formation is not synchronous with changes in the environment. Our results demonstrate unequivocally that in, A. irradians, stria formation is nondaily and is related to shell growth rate. The largest and oldest scallops (∼30 mm and 1.4 y old) formed striae at a rate of 0.17–0.2 stria per day whereas smaller and younger fast-growing A. irradians formed between 0.34 striae per day and 0.55 stria per day—clear evidence of nondaily and nonrhythmic deposition of striae in this pectinid species. Thus, striae cannot be used as a chronological marker with which environmental conditions can be compared.