Import prices fell 0.6 and export prices were down 0.5 percent from April, the bureau said.
From May 2012, import prices for all categories are off 0.8 percent and export prices dropped 0.2 percent.
The bureau said imports prices for of fuel fell 1.9 percent April to May, with prices 5.5 percent lower than May 2012. Non-fuel import prices dropped 0.3 percent in May but are 0.9 percent higher than May 2012.
Export prices for agricultural goods rose in May by 1 percent but remain 1.6 percent lower than May 2012. Non-agricultural export prices slipped 0.7 percent in May -- the third consecutive monthly decline -- and are unchanged from May 2012, the bureau said.
The bureau tracks import prices from various countries.
Import prices from Japan dropped 0.4 percent in May after declines of 0.6 percent in April, 0.2 percent in March and 0.5 percent in February.
Imports from China are showing the same trend.
"Import prices from China have not recorded a monthly increase since February 2012 and decreased 1 percent for the year ended in May," the bureau said.
Import prices from the European Union and Canada fell 0.1 percent and 0.9 percent, respectively, in May. Import prices from Mexico "ticked up 0.1 percent," in the month, the report said.