The population structure of the purple urchin and the giant kelp was determined along the Baja California peninsula in winter 2003 and spring 2004 using subtidal surveys at ~20 m depth in areas where an urchin barren and an adjacent kelp bed existed (the feed line where urchin harvest effort is usually directed).  Urchin density, and test diameter (mm) was measured in quadrants inside and outside of the kelp bed, as well as giant kelp density and fronds per plant.  We focused on four sites along the peninsula, two sites in the Northern state of Baja California, where urchin fishing is common (Isla Todos Santos and Campo Kennedy) and two in the Southern state of Baja California Sur, where urchins are not fished at all (Isla Natividad and Bah'a Tortugas). The latter two sites are very near the southern limit of the range for the purple sea urchin, , as well as for the giant kelp, in the Northern Hemisphere. A significant effect of both latitude and season on urchin population structure was found. A seasonal effect on urchin density was found, with a higher density in spring and these seasonal differences were exasperated in the North and were undetectable in the South. Kelp population structure (density, fronds per plant and fronds per square meter) followed the same seasonal pattern as urchin density and size (spring greater than winter, more exasperated in the north). We suggest that the populations of sea urchins and giant kelp are coupled along the Baja California Peninsula and may respond to climatic changes similarly.