An investigation has been made of the species of Sagitta present in parts of the Irish Sea, their relative occurrence, and the annual number of reproductive cycles of each species. Three stages of maturity as described by Russell (1932a) have been used. To determine the maturity the Sagitta were transferred to a solution of 75% alcohol and stained with borax carmine.

S. elegans Verrill and S. setosa J. Müller were collected in Liverpool Bay and in Port Erin harbour, Isle of Man. The former species was predominant at Port Erin. The coasts of Liverpool Bay have been added to the permanent range of S. setosa.

At Port Erin there appears to be but one chief spawning period for S. elegans which extends generally from January through May. Following May the young forms appear in numbers and gradually mature through December to repeat the annual cycle.

There appear to be two main breeding seasons for S. setosa in the outer Mersey Channels. The first and most noticeable begins in April and extends into June. There is evidence for a second breeding period in August.

Both species were found in the samples taken from Morecambe Bay and Anglesey. The stage of maturity of these agreed with the Sagitta caught at the same time at Port Erin and Liverpool.

The total monthly catches of Sagitta from Port Erin and the outer Mersey Channels have been recorded.