The distribution of the secreted protein ribonuclease T1 (RntA) fused with the enhanced green fluorescent protein (EGFP), RntA-EGFP, was visualized in hyphae of Aspergillus oryzae in the presence of a protein transport inhibitor, brefeldin A, cytochalasin A, or nocodazole. During treatment with the protein transport inhibitors, the distribution of RntA-EGFP changed and distinct patterns of fluorescence accumulation were observed. The addition of brefeldin A caused RntA-EGFP fluorescence to appear in reticular networks, and the disruption of the polymerization of actin filaments by cytochalasin A caused an increase in RntA-EGFP fluorescence intensity in the hyphae without accumulation in a specific cellular component. In contrast, RntA-EGFP fluorescence was distributed in different parts of a hypha during treatment with nocodazole, a compound that depolymerizes microtubules. In addition, quantitative analysis was performed using the RntA-EGFP visualization system to analyze the relative amount of RntA-EGFP secreted into the culture medium during treatment with the protein transport inhibitors.