Yalaku, a previously undescribed Ndu language from the East Sepik Province of Papua New Guinea, has two core cases (nominative and accusative) and four oblique clausal cases (locative-instrumental, allative, dative, and specific locative). The comitative case is used for marking an oblique within a clause, or as a marker of linkage within a noun phrase. In addition to Differential Object Marking, the choice of every core case and most oblique cases has pragmatic overtones. A further additional case-marking system is used if a core argument or an oblique argument is in focus (termed Highlighted Participant case). This principle, similar to that of differential object and subject marking in other languages, extends to the expression of possessor in possessive constructions. The coexistence of two independent systems of Differential Case Marking (based on different parameters) makes Yalaku typologically unusual. The appendix shows how language contact between Yalaku and the neighboring (and unrelated) Kwoma may have played a role in the development of Highlighted Participant case in Yalaku.