  We present 6.5-meter MMT spectrophotometry of 20 H II regions in 13 extremely
metal-poor emission-line galaxies selected from the Data Release 5 of the Sloan
Digital Sky Survey to have [O III] 4959/Hbeta < 1 and [N II] 6583/Hbeta < 0.05.
The electron temperature-sensitive emission line [O III] 4363 is detected in 13
H II regions allowing a direct abundance determination. The oxygen abundance in
the remaining H II regions is derived using a semi-empirical method. The oxygen
abundance of the galaxies in our sample ranges from 12+logO/H ~ 7.1 to ~ 7.8,
with 10 H II regions having an oxygen abundance lower than 7.5. The lowest
oxygen abundances, 12+logO/H = 7.14+/-0.03 and 7.13+/-0.07, are found in two H
II regions of the blue compact dwarf galaxy SDSSJ0956+2849=DDO 68, making it
the second most-metal deficient emission-line galaxy known, after SBS
0335-052W.
