Female biased primary sex ratios are known in many social spiders with highly structured populations and strong inbreeding. However, the primary sex ratio of the solitary spider Pityohyphantes phrygianus is also skewed, producing one-third males and two-thirds females, whereas the sex ratio is even in the related Linyphia triangularis. The female mating behaviours and genitalia were compared, revealing differences in, e.g. apparatus for sperm storing and in mating position; P. phrygianus takes a near perpendicular position before copulation starts. After completion of mating in P. phrygianus, experimental manipulations of the female's position for 24 h significantly changed the proportion of males in the first brood. This suggests that a biasing mechanism acting in the female's spermathecae-ducts is at least partly responsible for the sex ratio bias. However, the sex ratio in first and second broods averaged an overall mean of one-third males. The results suggested that females in the wild have options, which are not used, to direct the primary sex ratio away from one-third males. This deviation from Fisherian sex ratio may be caused by selfish genetic elements and/or meiotic drive.