An attached diatom, Arachnoidiscus ornatus Ehrenb., has been known as a fouling organism on the commercially important agarophyte, Gelidium elegans Kütz. in Japan. When Gelidium thalli heavily covered with A. ornatus were dried before shipment, they turned grayish green from red (original thallus color) because of the discoloration of A. ornatus, resulting in the reduction of their commercial value. In this study, the occurrence of A. ornatus on G. elegans was monitored seasonally and quantitatively by collecting G. elegans at three sites along the coast of Himi City, Toyama Bay from 2004 to 2006. Coverage of A. ornatus on G. elegans was measured using the scanned photographs (600 dpi) of G. elegans thalli. In 2004 and 2005, higher coverage appeared from summer to autumn (up to 20 to 40%), namely the harvest season of G. elegans, while coverage was lower in the winter and spring. In 2006, as our previous studies showed that common gastropods in the Gelidium beds never affected the abundance of A. ornatus, effects of water temperature and nutrients were examined by culturing the fouled Gelidium branches in surface seawater, deep (i.e., nutrient-rich) seawater (DSW) and three levels (25, 50 and 75% DSW) of mixed seawaters at 10, 20 and 30°C. The number of attached A. ornatus increased in 50 to 100% DSW at 10 and in 25 to 100% at 20°C but decreased in every culture medium at 30°C. These results show that the abundance of A. ornatus is highly dependent on nutrient levels and the temperature of the Himi coastal waters (ca. 10--27°C).