IF NOT NOW, WHEN? By Primo Levi. Translated from the Italian By William Weaver. Summit. 349 pp. $15.95.

GUERILLA WARFARE against the German forces of occupation in France, Italy and Yugoslavia is a familiar theme for the Western reader, documented by personal memoirs and recreated in impressive novels and films. Similar activity by Polish and Russian guerillas behind the German lines on the vast eastern front is less well known and Primo Levi's novel introduces an aspect of that huge and ferocious war which is entirely unfamiliar -- Jewish partisans. It is the story of the suffering, struggles and exploits of a small band of Jewish irregulars who fight their way westward, all the way to Italy, the staging area for Palestine.

Of course there were many Jews in the western resistance movements (Levi himself was a partigiano in Piemonte who was captured and sent to Auschwitz) but in eastern Europe, where the Nazi program for the extermination of the Jews was so efficient that few escaped the net, the situation was difficult even for the few that did, since the Russian partisans were traditionally suspicious of Jews and the Polish partisans openly anti-Semitic.

Levi's tale begins with the meeting of two Russian-Jewish stragglers from the Red Army; they find their way to a group of Jewish refugees camping out in the marshes. This settlement is attacked by German troops; the few survivors join up with a larger band and after successfully decoying a German parachute supply drop, reach a well-organized Russian partisan formation. "The Russians . . . looked at them uneasily, as happens when the unexpected occurs. In those haggard but determined faces they didn't recognize the zhid of tradition, the alien in the house who speaks Russian to swindle others but thinks in his own strange language." These Jews were warriors. "Each of them, man or woman, had a different story behind him, but searing and heavy as molten lead; if the war and three terrible winters had left them the time and breath, each could have mourned a hundred dead. They were tired, poor, and dirty, but not defeated; children of merchants, tailors, rabbis and cantors; they had armed themselves with weapons taken from the Germans, they had earned the right to wear those tattered uniforms." In their attacks on the Germans they "had found a new freedom, unknown to their fathers and grandfathers . . . which intoxicated them like the wine of Purim"; they had seen that the Germans "weren't made of iron, they were mortal, and when they realized they were defeated they ran off in confusion, even from Jews."

Later, threatened by German encirclement, the two groups split up; the Russians head east towards the front, the Jews (joined by one non-Jew Piotr, who had been with them from the start) break out towards the West, "to free prisoners, harrass the German rear and settle some scores," as their leader puts it. They move steadily westard through Poland, fighting all the way, to be finally overtaken by the advanced Red Army; the Russian authorities, non-plussed, ("Nobody has ever heard of Jewish partisans; that's something new"), intern them. They escape with a stolen locomotive, only to be rounded up again, this time on German soil in Silesia; but the Red Army unit in charge of the camp simply leaves in the night and they are free to resume their trek to the west. Their destination now (as for some of them it has been from the beginning) is Italy -- and Palestine. At the Brenner Pass they are spotted by military personnel from the Palestinian contingent of the British Eighth Army, who brief them on the procedures they must follow; the story ends in a refugee reception center at Milan, with the birth of a baby to one of the women of the group ("will he be a Polish baby? or Ukrainian . . .? or Italian?") and the news of the atomic bomb dropped on Hiroshima.

THE BOOK is fiction but it is firmly based on reality -- the stories of Jewish refugees arriving at an assistance center in Milan at the end of the war. "Some bands similar to the one I have aimed to portray," Levi writes in an Author's Note, "really did arrive in Italy; men and women whom years of suffering had hardened but not humiliated, survivors of a civilization . . . that Nazism had destroyed to its roots. Exhausted, these survivors were still aware of their dignity." He could draw too, on his own experience: his return from Auschwitz on foot through the chaos left by German collapse and Russian advance, (a journey recalled in his powerful memoir The Truce, 1963); he has also studied the documents, in English, Italian, Yiddish and German, which deal with Jewish resistance movements in the east. But there is nothing bookish about his narrative; Levi is one of modern Italy's most skillful and original writes. His characters are vibrantly alive; their voices resound with the unmistakable vigor and humor of east European Yiddish as we know it from such writers as Isaac Bashevis Singer; by some miracle of transformation Levi's Italian simulation of a language he himself knows "only marginally" has emerged as salty, vigorous English in William Weaver's fine translation.

This tale of resistance and escape, varying in tone from epic to picaresque, presents a realistic picture of partisan life, of a world in which the obsessive fear of encirclement, the fatigue and privation of the hunted, find their only compensation in the proud conviction that it is better to go to your death fighting than like sheep to the slaughter. Levi's partisans have a song; its refrain (adapted from an ancient rabbinic saying) explains his title: "If I'm not for myself, who will be for me? If not this way, how? If not now, when?" By Bernard Knox; Bernard Knox, author of "The Heroic Temper: Studies in Sophoclean Tragedy" and other books, fought with French and Italian partisans in World War II.