The growth of the postlarval shell of Lingula unguis on the north and east coasts of Singapore Island was studied. The youngest and the oldest postlarvae, observed over a period varying from one month to five years, grew in length and width all the year round. Length and width increments are inversely proportional to initial lengths and widths respectively, the linear negative correlation between increment and initial dimension being significant and high Growth is more rapid on the north than the east coast. Postlarvae transplanted to the north coast made greater increments than those left behind to grow in their original burrows on the east coast. Possible factors responsible for this difference in the rate of growth between the two localities are suggested. The regression of width increment on initial width nearly parallels that of length increment on initial length after one as well as after three years' growth, indicating a similar rate of decrease of increment with increasing initial width or length. The transformation of the shape of the shell is reflected in the changing width/length ratio. The change in width/length ratio during the postembryonic development of the shell is a continuous process. This ratio drops from a range of 1.5-2.0 to around 0.8 during the larval stage and to around 0.4-0.5 by the time the postlarval shell length reaches 4 mm. During the rest of the postlarval life the ratio varies sufficiently to make this an unreliable specific character for Lingula unguis. The region of maximum width of the two shell valves in the same individual coincides but its relative position changes during the long postlarval stage. The shell length of the female at maturity, the maximum shell length in a sample and the longevity of Lingula unguis, are discussed.