This study demonstrates that the interface between layers in 3D-printed polylactide has strength of the bulk filament. Specially designed 3D-printed tensile specimens were developed to test mechanical properties in the direction of the extruded filament (F specimens), representing bulk material properties, and normal to the interface between 3D-printed layers (Z specimens). A wide range of cross-sectional aspect ratios for extruded-filament geometries were considered by printing with five different layer heights and five different extruded-filament widths. Both F and Z specimens demonstrated bulk material strength. In contrast, strain-at-fracture, specific load-bearing capacity, and toughness were found to be lower in Z specimens due to the presence of filament-scale geometric features (grooves between extruded filaments). The different trends for strength as compared to other mechanical properties were evaluated with finite-element analysis. It was found that anisotropy was caused by the extruded-filament geometry and localised strain (as opposed to assumed incomplete bonding of the polymer across the interlayer interface). Additionally, effects of variation in print speed and layer time were studied and found to have no influence on interlayer bond strength. The relevance of the results to other materials, toolpath design, industrial applications, and future research is discussed. The potential to use this new understanding to interpret historic and future research studies is also demonstrated.