This thesis aims to identify the possible origins of the peoples who immigrated into the archaeological sites of Guajilar and Lagartero, located in the upper Grijalva River Basin region in southern Chiapas, Mexico, during the Late Classic period (AD 650-900). First, I present the Late Classic burial data from both sites according to four basic descriptive criteria: burial location, grave type, burial type, and grave goods. Then, I conduct a comparative analysis of the burial practices found at these two sites based on these criteria so that patterns in burial practices can be identified. Following the comparative analysis between Guajilar and Lagartero, I then compare their burial practices to those from two sites in the southern Maya Lowlands (Altar de Sacrificios and Seibal) and those found at various sites in the Guatemala Highlands (which border the upper Grijalva River Basin region to the north and east, respectively). The analysis reveals greater similarities in burial practices with sites in the Guatemala Highlands than with those in the southern Maya Lowlands. This suggests that peoples from the Guatemala Highlands were more likely to have immigrated into Guajilar and Lagartero during the Late Classic period.