Worldwide, a significant increase in watermelon (Citrullus lanatus [Thunb.] Matsum. & Nakai) growing areas has been registered in the last few years. In-row plant spacing has a significant effect on the growth and yield of watermelon, and can enhance competition for water and nutrients. The aim of this study was to investigate the effects of in-row plant spacing (1.0, 1.5, 2.0 and 2.5 m) and arbuscular mycorrhiza (AM) fungi Glomus mosseae inoculations on watermelon growth and yield under field conditions during 2003, 2004, and 2005 year. In 2003, the main vine length, number of leaves, and number of lateral branches were increased quadratically as the in-row plant spacing increased from 1.0 to 2.5. With an increase in the in-row plant spacing the early yield of watermelon decreased in 2004, while the fruit number decreased in 2003 and 2004. The total yield and fruit number decreased with an increase in the in-row plant spacing in all 3 yr; however, the fruit mass increased at wider plant spacings in 2003. Mycorrhizal inoculation increased the main vine length and the number of lateral branches in 2003. Compared to non-mycorrhizal plants, mycorrhizal plants presented higher early yield in 2005 and a higher early fruit number in 2003 and 2005. Mycorrhizal inoculation increased total yield in 2005; however, the fruit weight was not affected by mycorrhizal inoculation during early or total harvest. In this study, an in-row plant spacing of 1.0 m provided the best early and total yield while maintaining high fruit weight. The growth and yield enhancement of watermelon due to mycorrhizal colonization was not consistent; therefore, mycorrhizal inoculation could not be recommended as a standard production practice.