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Among 39 pentose-utilizing lactic acid bacteria (LAB) selected from acid-forming bacteria from the midgut of Eri silkworm, the isolate WX1 was selected with the highest capability to produce optically pure <span style="font-variant: small-caps;">l</span>-lactic acid (<span style="font-variant: small-caps;">l</span>-LA) from glucose, xylose and arabinose with furfural-tolerant properties. The isolate WX1 was identified as <i>Enterococcus mundtii</i> based on 16S rDNA sequence analysis. The conversion yields of <span style="font-variant: small-caps;">l</span>-LA from glucose and xylose by <i>E. mundtii</i> WX1 were 0.97 and 0.68 g/g substrate, respectively. Furthermore, <span style="font-variant: small-caps;">l</span>-LA production by <i>E. mundtii</i> WX1 in various glucose-xylose mixtures indicated glucose repression effect on xylose consumption. The coculture of <i>E. mundtii</i> WX1 and <i>Lactobacillus rhamnosus</i> SCJ9, a homofermentative LAB capable of producing <span style="font-variant: small-caps;">l</span>-LA from glucose clearly showed an improvement of <span style="font-variant: small-caps;">l</span>-LA production from 30 g/L total glucose-xylose (6:4). The results from Plackett–Burman design (PBD) indicated that Tween 80, MnSO₄ and yeast extract (YE) were three medium components that significantly influenced (<i>p</i> < 0.05) <span style="font-variant: small-caps;">l</span>-LA production using the coculture strategy in the presence of 2 g/L furfural. Optimal concentrations of these variables revealed by central composite design (CCD) and response surface methodology (RSM) were 20.61 g/L YE, 1.44 g/L Tween 80 and 1.27 g/L MnSO₄. Based on the optimized medium with 30 g/L total glucose-xylose (6:4), the maximum experimental <span style="font-variant: small-caps;">l</span>-LA value of 23.59 g/L reflecting 0.76 g/g substrate were achieved from 48 h fermentation at 37 °C. <span style="font-variant: small-caps;">l</span>-LA produced by coculture cultivated under standard MRS medium and new optimized conditions were 1.28 and 1.53 times higher than that obtained from single culture by <i>E. mundtii</i> WX1, respectively. This study provides the foundations for practical applications of coculture in bioconversion of lignocellulose particularly glucose-xylose-rich corn stover to <span style="font-variant: small-caps;">l</span>-LA.