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Being a widespread species, it's no wonder that Pelusios castaneus is quite variable over its range. At present Pelusios castaneus has no recognized subspecies, though for quite some time Pelusios chapini has been considered a subspecies, adding to the believe that Pelusios castaneus can reach a carapacial length of 38 cm.
There seem to be 3 forms of Pelusios castaneus. The "normal-form" which is often seen in the pet trade, comes from the countries Togo, Ghana and Benin. This form is recognized by the red-brownish to dark brown carapace and yellowish plastron with the borders of the plastron in the same color as the carapace, as are the bridges. The head almost always shows the typical dark reticulated markings. The overall body shape conforms to the typical body shape as can be seen in the description.
The "rain forest-form" seems to come from Nigeria and countries eastwards, and is recognized by its complete dark brown to black appearance of the shell, even the plastron is without yellow blotches. Specimens that arrived in Europe and believed to have been exported out of eastern Africa, are also completely dark colored. In contrast, specimens from Gabon are very dark in appearance but have an almost entirely yellow plastron and bridges.
From habitats including savannas and sub-Saharan areas originates a form that is called "savanna-form". It is recognized by the overall light coloration. The carapace is light brown to caramel and sometimes even showing large blotches of yellow, giving a marbled appearance. The plastron might show only at the extreme borders some dark pigmentation, but often the plastron is a completely unmarked yellow. The head often is completely unmarked, showing no signs of the typical reticulated markings. However, it seems that the savanna-form does often shows a different plastron form, often with quite a constriction at the abdominal-femoral seam and the posterior lobe often seems to taper, resulting in the anterior lobe being far broader then the posterior lobe. With the savanna-form the intergular scute also appears to be wider than in other forms.
Apart from these three known forms, there are also specimens known in captivity that show some sort of a dorsal stripe, like in Pelusios gabonensis. The dorsal striped Pelusios castaneus resemble mostly the savanna-form.
None of the above described forms are at present officially recognized and the entire Pelusios castaneus population in Africa should be investigated to draw conclusions concerning the status of the three known forms. It is quite possible that behind Pelusios castaneus are multiple subspecies.
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