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A new species of crayfish, Orconectes ( Crockerinus) alluvius, is described from headwater streams of the Crawford Upland and Mitchell Plain in southwestern Indiana. It occurs in small- to medium-sized sandy streams associated with leaf litter and woody debris. Orconectes ( C.) alluvius can be distinguished from other species in the subgenus Crockerinus Fitzpatrick, 1987 by a combination of the following characters. First pleopod of form I male with weakly developed shoulder on cephalic surface at base of central projection; central projection corneous, constituting 18.9-32.5% ( X̄ = 24.5%, n = 59, SD = 3.39) of total length of first pleopod, straight for most of length with a slight lateral curve at tip; cephalic margins of central projection feature impressed longitudinal striations distally on both mesial and lateral margins; mesial projection non-corneous with a slight outward bend near middle, ending in a flattened tip that is wider than the tip of the central projection; length of mesial projection subequal to length of central projection. The areola is wide, ratio 3.7-6.8 times of width to areola length, abdomen shorter than the thorax, and narrow chelae width. Mesial palm of chelae with three rows. [ABSTRACT FROM AUTHOR]