The gonadal development and morphometry of Appendicularia sicula (Appendicularia: Fritillariidae) were studied by means of histological sections and from the measurement of several characters. Based on morphometric measurements, the ingestion and clearance rates and the percentages of consumed chlorophyll-a were also estimated. The gonadal development is similar to that of Fritillaria pellucida. A small dorsal ovotestis, which originates ovary and testicle, shows large somatic nuclei inside. In the ovary these somatic nuclei are surrounded by a single layer of germ cells nuclei, whereas in the testicle they are found between the small nuclei of the germ cells. As the development proceeds, the large somatic nuclei are resorbed in both gonads, and the testicle finally envelopes the ovary posteriorly. After spermatozoids emission, the rectum is shown occupying most of the body volume. The length of the rectum shows a positive allometric growth in respect to the length of the trunk, which can be ascribed to its continuous accumulation of matter due to the lack of an anus. The relationship obtained between trunk length (TL, μm) and dry weight (DW, μg) was DW=2.24×10−11 TL4.098. Applying this equation, a two-fold increase in length would correspond to a 42-fold weight increase. For specimens with an advanced sexual maturity (400–450 μm TL), we estimated a mean autotrophic ingestion rate of 0.17 μg C ind−1 d−1 and a mean clearance rate of 5.01 ml ind−1 d−1. The percentage of total chlorophyll-a consumed by the A. sicula population was 5.13%. This value is similar to those reported for other appendicularian species in different temperate seas.