The biogenic amine octopamine (OA) and its precursor tyramine (TA) are involved in controlling a plethora of different physiological and behavioral processes. The tyramine-ß-hydroxylase ( tßh ) gene encodes the enzyme catalyzing the last synthesis step from TA to OA. Here, we report differential dominance (from recessive to overdominant) of the putative null tßhnM18 allele in two behavioral measures in Buridan’s paradigm (walking speed and stripe deviation) and a proboscis extension assay in the fruit fly Drosophila melanogaster . The behavioral analysis of transgenic tßh expression experiments in mutant and wild type flies as well as of OA- and TA-receptor mutants revealed a complex interaction of both aminergic systems. Our analysis suggests that the different neuronal networks responsible for the three phenotypes show differential sensitivity to tßh gene expression levels. The evidence suggests that this sensitivity is brought about by a TA/OA opponent system modulating the involved neuronal circuits. This conclusion entails important implications for standard transgenic techniques, commonly used in functional genetics. ### Competing Interest Statement The authors have declared no competing interest.