It is well-known that the intuitions which led Lukasiewicz to propose his system of three-valued logic were modal in character, having to do with the indeterminacy of contingent future tense propositions.' It is also wellknown that the truth-tables for conjunction and negation which he proposes appear to be inconsistent with any such modal interpretation. Specifically, Lukasiewicz adds to the usual values of truth and falsity a third value, intermediate between these two, which he calls "the possible" (but which is more accurately designated "the contingent"). He argues quite plausibly that when a sentence p has this value, its negation Np should also have it. The process by which he arrives at the table for conjunction is not explicitly stated, but in any event, it has the result that the conjunction Kpq is "possible" when both its arguments are.2 The objection is now immediate: although 'Lukasiewicz will be in Warsaw on Friday' and 'Lukasiewicz will not be in Warsaw on Friday' may both (on Monday) be contingent, the same can surely not be said of 'Lukasiewicz will and will not be in Warsaw on Friday', for the latter is always and necessarily false, on Monday as well as every other day. This objection seems so clear and decisive, that it might be (and often has been) supposed that there is no way at all to interpret Lukasiewicz's system modally.3 Surprisingly, however, this turns out not to be the case. On the contrary, we shall see that there is a translation of his system into the modal system S5, such that exactly those formulas are theses of threevalued logic whose translations are theses of S5. The translation is not without its own peculiarities, however.